sda is the USB stick I am currently booted on
nvme0n1 indeed contains my LUKS-encrypted system partition
The 8GB partition of nvme0n1p2 is a swap partition. I stupidly loaded it on /dev/mapper/cryptboot, but that’s my mistake
I am indeed greeted by a LUKS passphrase, which works and then brings me to a grub menu with no Manjaro entries.
Thanks again for your help!