Put the microwave on a "Jackery" battery back-up or similar.
My microwave and frig are on the same 20 amp circuit and my Jackery (1,500 continuous/3,000 peak) can run them both. The time you run the microwave won’t deplete it and it continually recharges from the wall.
Plus, if you ever have a power failure, you have the Jackery for back-up.
And, they are nice for camping and anytime you may want to be able to use a small appliance where there is no power available. Even nice for things like soldering in the yard without a long extension cord trailing along.
You can even get solar panels to charge them.