Switching a 11 speed cassette+wheel with a 8 speed freewheel+wheel. Any compatibility issues?
No sorry, this is not practical.  Freewheels mostly stopped at 7 speed, and 8 speed was rare because of excess unsupported axle causing bent axles due to leverage.

Your suggested plan would require fitting 8 speed shifters and an 8 speed rear mech.  That's too much faffing about when you want to swap wheels.

Instead, you could find an 11 speed hub with the same spoke count, and buy an 11 speed cassette.  With luck the donor wheel's spokes will suit, and you can reuse the 8 speed wheel's rim in a new wheel.

The front wheel should be a direct swap, other than checking brake pad alignment.