1. Download this (Mantis LOD Editor Professional Edition), import into your project, then download this free non-destructive tool that uses it - Booth Link and import it afterwards

2. In Unity, add "NDMF Mantis LOD Editor" component to each object in your hierarchy that you want to edit.
   Then press "Preview" and move the "Quality" button until you're satisfied with the ration of Quality/Optimization, also feel free to play around with other settings
   

This saved one of my avis I thought to be unsaveable in terms of triangle count:

Good practice:

1. Don't put multiple clothes/hairstyles on one avi. With most modern avis being triangle heavy from the get-go and VRChat's abysmal (for modern PCs) 70k triangle limit you already have very little headroom to add your customizations, please distribute different outfits/hairstyles between separate avis and just switch between them when you feel like it (it is less convenient, but it's a small sacrifice)

Technical stuff:

1. https://github.com/d4rkc0d3r/d4rkAvatarOptimizer - slap this shit onto your main avi, set it to full, check if anything breaks in Gesture Manger (it most likely won't, across numerous avis it only broke something like one time)

2. DELETE MESHES OBSCURED BY YOUR CLOTHES!!! https://vpm.anatawa12.com/add-repo - add this to your VCC, then in your project enable the "AAO: Avatar Optimizer" package. Add "AAO Remove Meshes by Blendshape" component onto whatever is obscured by your clothes and checkmark everything you want removed (it is non-destructive, it won't ruin your avi as it only applies on upload). This will shave off a good chunk of triangles off of your avi
   It also has a lot of other functions, but I've only used it for one day so I can't tell you about anything else.

3. https://vpm.thry.dev is pretty self-explanatory, you add it, go to Thry -> Avatar -> VRAM, put your avi into it and REDUCE ALL THOSE FUCKASS 4k TEXTURES! You won't need them, do like 2k max (I would even recommend 1k for smaller parts like gloves/shoes/eyes/hair) until your Texture Memory is at least Poor or better

4. (Not my area of expertise) in Blender, you can import an FBX of an object and add a "Decimate" modifier to it, then set the ratio to ~0.5, which will shave off a good amount of triangles. One problem tho: it destroys your Blendshapes. I'm practically brand new to blender and idk if it's fixable, so I only use it on objects that don't need any blendshape modification

5. UPD. Refer to my next message in this thread, there is an extraordinary tool for significantly reducing triangle count that you should always use!

So in conclusion: d4rk optimizer will resolve most of your issues, AAO will help you with triangles (usually main issue in avis), Thry will help you easily reduce your texture size so you don't kill everyone's (and your own) VRAM.

PLEASE CORRECT ME IF I'M WRONG OR ADD SOMETHING THAT I FORGOT BECAUSE I'M IN NO WAY, SHAPE OR FORM A PRO AT CREATING/OPTIMIZING AVIS! I will edit this if I remember anything else.

Also there's this post, I only quickly skipped thru it but I think it's very similar to what I described. It has nice video demonstrations tho! (https://x.com/duinrahaic/status/1864819572596969874)

https://vpm.thry.dev