Tanks inc fuel sender

The reason Mopar and others use thermo electric guages. They do not react instantly like your aftermarket guage is doing.

You could probably come up with some type of dampening device that would average the level over time and feed a constant signal to the guage.

Maybe even a meter match might do that.

I'm no electronics guy but even the right capacitor might smooth out the rapid shifts.

Another thing that comes to mind is I have seen up and down type senders with a plastic tube around them, the tube has holes in it and fuel can get in and out but much slower than the sloshing tank