What can feltrite be used for?
Feltrite can be used for 3 things, all of which you've already identified:
- Upgrading skills
- Upgrading weapons
- Healing yourself
Do note that there are two DLC packs coming out later in 2019, which will add more weapons and abilities to the game, and thus more things to spend feltrite on.
Sources:
What is feltrite for in Rage 2?
Rage 2 guide: Currencies, upgrades, and components
Rage 2 Tips: 16 Things the Game Doesn’t Tell You
- 2019 Rage 2 Roadmap