New Tumbleweed snapshot 20241127 is out and it downgrades plymouth related packages (and upgrades a lot of others).
Because of what i read in Update hung up on post trans update - #32 by mendres82, to avoid the possibility of post installation (posttrans) scripts getting stuck at n% I ran first in tty as root systemctl stop plymouth-start.service and then upgraded Tumbleweed to 20241127 snapshot. Upgrade and posttrans scripts ran without any errors.
Chose Reboot in Plasma/Wayland session and pressing Esc key when shutting down I could see system messages already. Also when computer was booting, and I could decrypt /home without any problem again.
Tomorrow I’ll upgrade another computer without stopping plymouth service first to see how it goes.
PS: I didn’t dare to install your possible fix as it affects grub. 
Sorry, @arvidjaar!