Built in microwaves generally are higher wattage and thus the seperate
20 amp circuit requirement.

You have a choice

go back to a countertop microwave

Do the job right and run a new 20 amp circuit with 12 gauge wire.

or hack a work around and live with tripped breakers when someone
forgets you cant use microwave while using any other countertop
appliance.

Personally even a day of work is better than a lifetime of
inconvenience

In preping for a kitchen remodel in a 60 year old home I added a bunch
of new circuits dead ended in work boxes on basement cieling, to make
later kitchen job easier

There are special long bits for electrical work they are 3 feet long
and extensions are available you can drill directly from hole in wall
behind new microwave to basement, just use care watch for obstructions
like water lines and other power lines, a small mirror in hole helps