I don't know if it applies to modern microwaves, but years and years ago, I learned a secret to keeping a microwave running long after its normal lifetime, and without paying a professional repair person.

It would start arcing eventually, and the solution was to "replace the waveguide". Someone not physics-illiterate could explain more about how that functions, but practically speaking, it involved getting a flat sheet of special material (mica I believe?) and cutting a small panel out to match an existing one on the inside of the cavity.

Looks like a big sheet is <$10.