What I do when I convert a model to be quest compatible is usually done in this order:
Fix any material shaders to be quest compatible, if the material has a greyed out shader do the others first and come back to it, make a new material and apply the texture and anything else needed to it then drag n drop it onto the object using the material.
Texture optimization. Prioritize the least detailed or least seen textures. All textures that are 1 color get set down to 32, 2 colors are 64 or 128 depending on how visible they are, 3 to 5 colors are 256, and 6+ colos are 512. Body textures are either 512 or 1024 depending on how detailed they are. If it has a lot of patterns and tattos that have lots of small details then use 1024, if it has none of those and is just a simple skin tone then use 512, or even 256/128 depending on if it includes the face or not (256 if it does, 128 if not. Up to you). Hair textures should range from 32 to 128, no higher.
Physbones. Use Anatawa12's Avatar Optimiser to merge physbones where possible. Prioritize merging boob or ass physbones if they are listed as seperate components. If all the physbones are under 1 object for some reason, then make new items in the heirarchy to hold each individual physbone for merging. Make sure that both physbones you're merging have the same paramaters or it'll give you an error. The multi child type needs to be set to ignore on both of them as well. Merging physbones decreases the component count, but increases the affected transform count, so keep an eye on that in the statistics menu that shows up when you enter play mode.
Check for any errors or anything that might be broken (textures, prefabs, gestures, shaders, etc.) in play test mode and click on them, figure out what's wrong and exit play test to fix it. Do this for one problem at a time, it may be time consuming but it makes it a whole lot easier. If the problem is something that is not able to be fixed no matter what, delete whatever it's from if possible. If you can't delete the thing its from, try deleting the specific object causing the problem, and if you can't do that then ask someone for help. Don't be shy.
Upload and make adjustments where needed. Don't make any adjustments too big, and make sure not to close the project until you're absolutely done so you can undo if needed. And Save regularly while making adjustments, but don't save on the same file, make different files for every few edits or adjustments so you always have a spot to backtrack to if you mess something up.
Enjoy! (let me know if ya have any questions)
