I've installed noVNC on a Pi before, didn't try accessing it through an iPad and I don't recall if I was using desktop or just terminal, BUT, I think something like noVNC could get you close to iPad-as-RPI-screen. Add in some VPN functionality and you can keep the Pi running at home, but use the iPad anywhere you can get online (though now you're getting into diminishing returns).
I use my iPad Pro as a remote terminal to an RPi 400 and an Odroid N2+. Both of these machines are running xrdp, and I use the Microsoft Remote Desktop client on the iPad. RDP seems somewhat faster than VNC.
RDP is much faster than VNC because it has improved compression and latency management. Also, it is its own X server generally, and not a bolt-on, so less overhead.