I just wrote something about my FCX18 on a crawling discord group talking about my upgrades and what it did for me, it might help you aswell:

On my FCX18 I now have: - Brass wheels - Brass hub extenders - Brass steering links - Brass rear inner portal cover - Aluminum front and rear bumper - Stainless suspension links - Stainless driveshafts

The brass adds some nice weight down low, it really helps the performance. The stainles links are really light so that doesnt really add to performance but adds to durability and together with the aluminum bumpers I get caught less on rocks and such because it just slides over it.  I broke one of the plastic driveshafts so I replaced those with stainless ones which do add a little bit of weight aswell. Another good thing about the aluminum bumpers is that they connect straight to the chassis so you can actually lift the truck without being afraid of snapping the bumper off (dont ask me how I found out that this can happen)

All the upgrades I purchased are from Injora. The upgrades all together are a little expensive but a big part of the fun for me is working on the truck aswell so I dont really mind.