Yeah, they need to be edited individually as not all vehicles will have the same upgrades.
You'll want to edit the 'CAmbientModelSet's with '_MODDED' in the name, but you can edit whatever 'CAmbientModelSet' you want & even add new vehicles (add-ons etc).
Practice with the 'LS_CUSTOMS_MODDED_SP' 'CAmbientModelSet' as those vehicles appear parked outside the LS Customs in Burton (the one near Rockford Hills, see pic below) so are easy to find & check the results.
Just drive around the block & back in to that^ area to get the 'LS_CUSTOMS_MODDED_SP' vehicles to respawn/change etc.
Before editing 'vehiclemodelsets.meta' I would advise making a backup of the entire 'update.rpf' archive, not just the 'vehiclemodelsets.meta' file.