Edits, but here are a symbiote and nanite model for the "race", such as they are.
Symbiosis:
The Symbiote has its own Will score (either), which can be made a strength if you choose out of your usual pool. When an event happens that requires the symbiote to take over completely (you lose a battle of wits in a conflict of interests, it's simply more experienced, etc.) it deals you 1x Tool damage (for the strain on your body), and 1x Will damage if you don't consent to the action. You always roll disadvantage on charisma checks (since you're a composite - can be avoided if one takes over with Will, but resets next time they see you as a composite) and folks who know what you are naturally look upon you with concern (until they know better). Since the symbiote has a vested interest in keeping you alive, treat your milestone as one higher for the purposes of calculating health.
So, acknowledging the symbiote with a new mechanic, a negative, and a boon.
Nanites:
Start Body (if enhancing existing abilities) or Tools-Supernatural (if learning new "weird stuff") at a d8. You now have 2 strengths left. Electricity and radiation do 1d extra damage (since they effect the nanites). When you hit half health (and all damage moments after), roll Will-Leader to see if you can power through your body glitching. If you fail, you fall unconscious while the nanites glitch, even if you have health left.
So a stronger being, but it comes with risk.
Thoughts?