Idea: boot from the encrypted partition, unlock the encrypted partition after login to have access to the private data.
I don't understand how you can boot from the encrypted partition and only unlock the same afterwards. Assuming you mean to boot from the unencrypted partition then yes, that is possible as from any other USB stick as Jan mentioned. My setup does exactly that (GRUB installed on nitrokey storage).
Nitrocli should work for the use case of opening the encrypted volume. Note that it currently relies on gpg-agent (and with it pinentry) to ask for the PIN and cache it (simply because that's fine in my setup). That should not be a problem but if it turns out feel free to open an issue and I can address it (I am incidentally the author of that tool ).