I'll consider using a bigger microSD card to save recordings, I hadn't thought about this option.
That option have advantage and disadvantage at the same time. I prefere to keep my media file on a separate drive (USB stick in my case). That could be a major advantage when needs to reinstall the whole system (somethings went wrong, major system upgrade). So I recommend to think it twice to keep your media/personal files in the same drive (microSD) as the LE system. Maybe if you have a "not too slow" USB stick, you could try it for TV recordings. If you will go with TVheadend, it's a bit of work to set it in webinterface, but after that it's very stable (at least my experience).
Another thing, I'm using at an RPi3B+ with 3 USB tuners (as TVheadend server, just watching, without recording) for years without any serious issue. I think the streaming from tuner isn't need too much RAM, probably for recording is the same.
How did you inter-connected Xbox tuners?
Did you mean about antenna connection? Use a simple RF antenna splitter, something like this: