struggling to use MOSFETs as switches: is the problem my circuit or the choice of mosfet?
You need 5V above the source of the MOSFET.  If the FET were ON, the source would be at 5V and you would ideally have 5V ABOVE the source on the gate, or 10V on the gate.

You could use P-Channel FETs and pull the gates to ground to turn them on, or you could provide a higher gate voltage for the N-Channel FETS.

The low side FETs should be switching OK.