sorry for late reply, @anon31687413 but only yesterday had the chance to experiment a bit…after some attempts with grub (following @ricklinux Everything About Grub! post), I decided your approach with good old bios/mbr is definitely the best in my case.
I confirm Calamares handles it gracefully, and it’s even able to correctly perform encryption on both boot partition and main btrfs volume.
thanks for your advice!