Race-specific crafting is a base game thing and easy to implement. Could add alternate healing items that require some amount of an Isotope (Like Plutonium) as a racial craft. Given you also need mass amounts of those for ship/generator fuel and a bunch of other random crafts, it seems like a reasonable solution.

Alternatively, could invert how radiation damage works for them, such that they heal from the DoT instead of suffering damage. That means Fission generators also serve as regeneration beacons and you could carry nuclear waste with you to make improptu healing zones. Still akward and not easy in practice, but at least offering some scaling options beyond their special seed. It's not Radiation Damage is a particularly common weapon element, and they're already super resistant to it in the first place.

That said, it does outright refer to them as a hardcore race. Can't think of a way to make Starbound more hardcore (and irritating) that being unable to heal after getting jumped by high-tier soldiers with some of FU's nigh-broken guns. You know what you're getting into when you pick them.

Xi have their own healing items, crafted in their crafting table. Their tiered options compete directly with bandages so you are not left in the dust with healing. It sounds like you haven't explored that table, especially since they have other gear at it to help Xi get around, heal faster and such as well.

Xi healing zones can be improved by wearing relevant Xi gear that beefs up the healing rate, and can be stacked.

X'i are intended to be hard, requiring a new approach to gameplay. In this they do admirably! But by not exploring their racial crafting you've neutered yourself.