Oil slosh

My oilpressure will momentarily drop to 0 upon hard braking when the oillevel is near/just below the 'Fill'-mark.
Doesn't happen when it's on 'Full' or slightly above.

You can always take note of the oillevel on the stick, drain the oil and measure the amount drained. Then add with a bit extra until the pressure stays good (which you'll only know when you install an actual oilpressure gauge).
Maybe better option is to add baffles inside the oilpan to control the oil better if you plan 'playing around' more often.