Like @ClockworkOcean suggested it sounds like landscapes isn’t what you need. I would suggest looking into distance meshes, think like landscapes but as a simple static mesh with a material on it. Much cheaper performance wise and you get the 3D.
Not sure I would recommend HDRIs unless you’re going for a very old school lighting setup as HDRIs has a lot of baked information that will clash with a real-time lighting solution.
Regarding your problems running out of memory, this is a known issue in 5.1 and has been mostly fixed in 5.2. Memory usage as a whole has been very poor in UE5 but getting better now.