As with nearly all things Unix/Linux, there is more than one solution.
You mentioned you want to be able to launch your server as a foreground process but be able to send it to the background and ... recall it to the foreground at a later time. I'll address answers to those questions here. There is a completely different way of doing things if you want to ...
I ran into the same issue, here is what I did:
CTRL + ALT + F1
Find running processes
I found the TTY7 process PID number then enter command where nnnn is the PID number.
sudo kill nnnn
Go back to GUI (TTY7)
ALT + F7
Login to your GUI session, this will be a new session.
There may be a better way to do this but this saved me from the pull ...
i'm running with rpi4 4gb (allocating 2.5gb ram), runs fine for 4 people, although at times i'm still getting the "running xxx ticks behind error" in the console. I've got a 54Pi knockoff fan, which at keeps the temp from 40-45C. I've overclocked to 1.75ghz, running fiber optic connection, so far pretty good. will try to run with a few more players soon!
I came to this thread because I was having the same issue, only I couldn't select or type anything, much less Ctrl+Alt+F1. Esc happened to work for me, taking me back to the main screen, from which I could X out.
I have a Raspberry Pi 4 running Minecraft 1.15.2 and it works just fine. You may want to set the preset ram to 1.5 GB and the max ram to 2.5 GB in your startup script. If you would like to watch a setup video, click here.
Extra: If you want to download a spigot server, click here.