This is going to be a pretty long post, I wont cover all the Trinkets but just the ones I’ve used and how I feel about them (some of them extensively, some not as extensively).
Olympian Frame - This trinket seems like it’d be good if you’re going for a build that involves prop killing zombies. I haven’t figured out a good way to do this mechanically or easily, so I stopped trying to go for a build that just uses big props to hurt zombies; most maps where you can achieve this often have to be quite large and have a lot of area to allow it to happen, with few humans around.
The other use for this trinket is obviously to just throw props to higher areas or closer to barricading spots. The strength of throwing props initially is already strong enough for such a purpose, so this Trinket isn’t that useful, but imo has potential for a prop killing build if mechanics are ever changed again to allow for such a build to happen.
Mining Goggles - IMO outclassed by the Night-Vision Goggles, which are also worth tier and if you have 10 worth left, you’d probably get them over paying points for Mining Goggles. These also don’t have a great effect, and as a counter to Doom Crab (be honest, there isn’t anything else you’d ever get these for) they cut the effect in half but he can reapply it too quickly for it to ever matter. I don’t know what else to say about these, they’re just not that great in my opinion. This is perhaps the worst trinket in the game as it’s simply outclassed.
Power Gauntlet - The effective is very good; the reliability is kind of low however. The issue I have with the trinket is actually chaining together the hits to get the effect; how worth it is the damage boost? Usually before I ever get to the 5th swing everything I chop is dead unless they’re literally swarming the barricade; you can miss one time because everyone with guns kills everything in front of the barricade, and the swing wind-up time is often quite large on late game melee weapons so it’s a little tricky to get this to work.
I don’t think it’s bad still, but it’s just a little unreliable.
Reload Speed Trinkets - I often don’t find myself getting these trinkets due to Quick Reload being a thing. That’s about it. It might stack but the reload speed bonus from the skill is often enough by itself to really get things done.
Vit Pack III and Targetting Visor III - It’s hard to get far enough into the game to make these trinkets worth buying, but they are very good. It’s a balance between spending your points to upgrade your gun or get ammo, and trying to get trinkets; the guns and ammo are often more beneficial than the trinket for these ones in particular.
They’re not bad however, perhaps a bit overcosted but again they’re quite late game in terms of trinkets.
Necrotic Senses Distorter - I often see this picked if the Boss Drop is dropped outside the cade and zombies are camping it; with the right build you can zip out and pick it up without them noticing. Another use is for scavenging slightly more safely when near zombies.
A good player will always notice a human running around even without their aura though, so yeah I assume most of the picks are for the Socom (and I’d imagine most of them are through scrap purchases, but I could be wrong).
Cardio Package I, II and III and Hemo Adrenal Converter I, II and III - I’ll start by saying that these trinkets are really good but mostly for close quarters or melee builds. They are made worse by the fact that Blood Armor is a Skill, and 10 Blood Armor is generally enough to ward off poison/bleed or chip damage. These are far more useful on Wave 6 because of Chem Zombie burst damage; the issue is you need a lot of scrap or points to warrant getting these trinkets.
Hemo Adrenal Converter is easily abusable with the Longsword to gain consistent amounts of blood armor whilst dealing a lot of damage to everything attacking the barricade. Damage Resistance makes this damage lower, so you’d be hard pressed to find the scrap to upgrade both your weapon and your trinkets.
Of course you can use points to buy trinkets; I think the point cost of a lot them is quite steep. I will have to look into a melee-focused build again, but it’s often very map specific and most of the time too slow to acquire points if barricades are not suited for melee weapons. I only usually go for Cardio Package II and Hemo Adrenal Converter II (as well as having HAC I from Worth Menu) as 20 blood armor is often enough to not get destroyed late game, and you gain enough blood armor from hacking away with the longsword, which is a super safe option and gives very good damage output.
HP is often a better resource than blood armor is at the moment. Increasing your health and health gain on heal makes it easier for medics and easier for you to do things. Blood Armor is quite irrelevant due to how low the amounts you have are, except for chip/dot damage.
Also you have things like Vitality Package I - III, Anti-Toxin Package and Kevlar Underlay which help if you make a bad decision and get hit, which hurts the purchase amounts of Cardio Package and Hemo Adrenal Converter (why buy these if you can just prevent the damage altogether by a set amount, or just have more HP in general which is a better resource).
Even though these trinkets aren’t picked that much I still think melee is in a good spot overall from testing and trying it out, and they are still very useful if you’re using them properly; atm they get rid of chip damage very easily as a melee built character.
Arsenal/Resupply Pack - These are often very good trinkets to get but usually I only find myself getting these from a remantler if the team happens to have a Remantler, but no Arsenal/Resupply or only a Resupply and Remantler and we lost the Arsenal Crate (this is supported by the statistics).
I think the biggest issue is that you can’t easily tell who has one due to transparency levels and things like this; it makes it very easy to lose the person who has them equipped at high player counts and having a non-transparent Resupply or Arsenal Crate is much better. Perhaps highlight players who have Arsenal/Resupply Packs equipped somehow visually (idk how you’d do this, but it’d be handy).
Oxygen Tank - I often get this so I can scrap my starting weapon and buy an extra amount of ammo for my T2 gun later on, or to upgrade my Melee Weapon if I’m going that route (it reduces the amount of things i have to find on the map to scrap by 1 too). This is generally very beneficial and 5 worth for 1 scrap is very bargain especially for builds that don’t rely on debuffs or worthiness to extend the worth amount you have initially.
Boxing Training Manual, Ammo Vest I, etc - No idea why Boxing Training Manual is picked as I don’t catch anyone landing a stun combo literally ever.
Ammo Vest I is just a DPS buff but I think it’s too minor to really do much; you could say that if people aren’t picking night vision or a build related trinket (deployable reduction time etc) they’d just pick this since the 2% can help with dps to a small extent. I personally don’t pick this but have noticed it picked a lot.
That’s basically the summary of my usage with trinkets. The ones I didn’t mention I’ve either never used or haven’t found a good purpose for them (stuff like Pulse Impedance Module seem good, I just haven’t used them yet, whereas Feather Fall, Curb Stompers etc all seem very bad except for niche situations that don’t happen that often). The effect simply doesn’t come into play often enough to help.
EDIT - Momentum Support System I II III - This whole line of upgrades seemed okay, but it’s hard to fit it into a melee build. There are too many trinkets to consider when making a melee build, and you will never have the resources to buy all the trinkets you would need; these are by no means bad, but having the additional chip damage protection is incredibly useful, and hemo adrenal converter + cardio package is already 2 lines of upgrades required to effectively melee at a barricade (technically you don’t need either, but it makes it much safer on Wave 6 to melee within a barricade).
Like you said in your post, merging existing trinkets together would be beneficial to solving the problem of having to buy whole sets of trinkets; overall I don’t think trinkets are useless but you certainly don’t need a build stocked full of them to succeed or anything like that.