Today I removed Win10 from nvme, because of bug:

So I decided to remove such unresponsible hardware-destroying OS, and put Devuan on nvme and get Steam there.

Installation with (not-LVM) Encrypted root partition with unencrypted /boot, EFI system partition, installed from desktop-live - does boot.

However, there are some errors You see immediately on booting, that, btw, do not prevent booting:

error: no such device: <UUID of decrypted root appears here>
error: file '/usr/share/desktop-base/grub-themes/cinnabar-grub/theme.txt' not found.
error: file '/usr/share/desktop-base/grub-themes/cinnabar-grub/unifont-regular-16.pf2 not found.

I assume first error is because GRUB can't see UUID of decrypted root_fs before decrypting.
This is correctable by editing grub.cfg lines 63, 65, 81, 83 and replacing that UUID with uuid of /boot partition,
But, it will revert on any grub-updating. I do not know how to make this change permanent on grub update.
Any help on this will be appreciated.

Second and third error is - grub themes are on root_fs which is in time of showing this message, stil encrypted,
so I copy/pasted themes on /boot/grub and corrected paths in 05_debian_theme and /etc/default grub ... and ...
... and after that noticed that is already discussed here 

It seems that grub-pc package no longer runs a configure dialog when you install it with 'dpkg -i' which is what the desktop-live wants to do for a bios install. This doesn't seem to be a problem with unencrypted root partition and a single hard drive (virtual). In that case, grub gets installed to the MBR of /dev/sda without asking you. I don't think it's a problem with full-disk encryption, but it's been at least a couple months since I've done one of those.

There are two possible solutions:
1. Use the cli installer instead of the graphical. Run 'refractainstaller' from a root terminal (or with sudo).
or
2. If you really want to use the graphical installer, edit /usr/bin/refractainstaller-yad to uncomment line 1762 to enable the choose_grub function after the grub-pc package gets installed.

I think someone may have reported this when we were testing the betas, but I don't feel like looking for it right now. I did file a bug report: https://bugs.devuan.org/cgi/bugreport.cgi?bug=485