As far as using stock GTA V. the way you described is the only way to edit weapon icons. If you use my script mod "Pickups" (https://www.gta5-mods.com/scripts/pickups) you can edit each icon individually as well as add new icons for added weapons. You'll notice the Fire Extinguisher has an icon when using the custom Weapon Wheel in Pickups. Normally it does not.

Lol, I know the feeling, man. I remember trying convert models from Bioshock and Halo 3. Never did finish them either.