Hi @linux-aarhus – thanks a lot! Especially for the clear instructions (see my OP: from the “make extra boot”, which people sometime advised, I did not understand that you need both efi and boot partition and therefore ONLY created an efi partition w/ boot flag in my attempts… I assumed that this was meant by boot partition).
I might try it again, although I now already used your instructions Install Manjaro using CLI only and https://archived.forum.manjaro.org/t/howto-install-encrypted-manjaro-using-cli/110553 in parallel yesterday and I think I managed to set it up as intended. (using 500MB ext2 for /boot – any issues to expect compared to 1G ext4?)
It is faster now than with encrypted boot(/efi?), though still “kinda slow” compared to the, e.g. POP!_OS that I installed w/ encryption on an older computer. On my current PC, booting Manjaro with only encrypted root now still takes some 15 s longer than booting unencrypted Mint 
Yes - previously the decryption phase was a long wait - I think this has been resolved with the latest iteration of the grub boot loader.
Cannot confirm. Just checking the “encrypt system” option of the installer added a 50 s wait time only for decryption (directly at the POST screen), compared to ~5 s when decrypting the same disc with a live system (i.e. not with GRUB). Thus my post here 
Is it possible that the maintainers of the Manjaro Cinnamon edition did not implement these changes that resolve the issue yet? Not sure whether this is helpful but the disk was encrypted with luks1 when using the option of the installer.