I can't seem to get the base leveled and would be happy about tips.
The ultrabase seems to warp the H shaped frame when I tighten/loosen the leveling screws.
When I level one corner, move to the next and then move back, the leveling is off again.
This is so frustrating that I am considering either changing the H frame or selling the printer.
Get yourself an level inductor sensor and a glass mirror - you put aluminium foil between the Ultrabase and mirror so the sensor can sense the level - you will have to enable bed levelling in the firmware - there are plenty of tutorials on Youtube - you won't be bothered about levelling issues and the mirror glass gives a nice looking shiny finish.
Difficult to say...I do not have that warping problem. Maybe your bed screws are so tight. In my opinion, you should not care about the H bendind if your bed does not.
Anyway, the best solution could be to install an autoleveling sensor, like BLTOUCH or similar, and you will not need to level your bed anymore.
I don't care about the frame
The problem is that it is really impossible to level it. You turn the screw a little bit and the other screw is off again.