Switching a 11 speed cassette+wheel with a 8 speed freewheel+wheel. Any compatibility issues? I'm trying to get a pair of training wheels to swap out for my race wheels. I dug up a pair of freewheel compatible wheels and I'm trying to buy freewheels now to fit the bike. The bike's drivetrain is 11s (Shimano Ultegra) but it looks like freewheels only go up to 8s. Would these be compatible with my bike? If not is there an easy way to make them work?
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.