I had similar problem, systemd not showing in boot menu, UEFi completely ignoring boot order from efibootmgr and changing it back. Tried to fix it for 2 days and didn't find a valid solution, then suddenly i've seen there are some boot options in my UEFI, they were grayed out bc secure boot was disabled. I enabled secure boot to check what i can do and used the option called "select an UEFI file as trusted for executing" and i've choosen systemd. What happened? UEFI added systemd option to my boot menu so i disabled secure boot and raised the priority of systemd in boot order and it works. Total luck, you might try this.
TLDR: Go to UEFI -> enable secure boot for a while -> use "select an UEFI file as trusted for executing" -> choose systemd. This should add systemd to your boot menu. -> give systemd a priority