I would replace the on / off switch and inspect all the wiring for black or green wire disease.

Test the control surfaces against some manual pressure, noting that the servo output shafts and everything else is slop free.

Fuel creep can make the firewall connection to the rest of the plane questionable, I'd give it a tug after the engine is mounted.

The servo rails and wing hold down points should be tested against some manual pressure.

Might as well press and pull on anything else that could pop loose.

Check the plane for straightness, too.