Very good! The idea with only the boot partition in cleartext would be fantastic. No one provides this so far, you would be the first and only!

I am not a Linux hacker, but why not try to cooperate with the guys from Jondos who seem to have deep linux knowledge: https://anonymous-proxy-servers.net/en/software_more.html. By the way, they reside in Regensburg and also are idealists. If they cannot help they at least might know someone who can. Or try the tails guys: https://tails.boum.org/

Final questions:
1) Do you plan to integrate U2F into the Nitrokey Storage?
2) When will the Nitrokey Storage be available?
3) How much will the Nitrokey Storage cost for the respective sizes?
4) What is your recommended backup strategy with the Nitrokey Storage? A second key (could get expensive) or also another approach?

Keep going!

Fill in the blank.