IMO I would never have a builtin MW they do not last forever and you already have found that sizes are hard to change .I have no issue with MW on the counter where you use it and when time comes to get anew one it will fit. I do not like MWs under the counter at all have you maybe looked at a shelf maybe under a wall cabinet or in apantry with an open space for the MW

Well the blessing and curse of these fora is that you will get all sorts of varying opinions :) I am personally loath to have appliances on my counters. I think it is a reaction to surviving the chaos of raising kids and the clutter that goes with it. I just want a nice expanse of smooth, luxurious countertop, which I can clutter up with my baking and gardening projects, and then clear off again to return to luxurious and smooth...Consider splurging on the model that is made to be under the counter, rather than fitting a countertop model into a 24-inch hole. (because as you note, said countertop model will need to be smaller to fit). I would not want to give up drawer space just to avoid a built-in model!

We had a 24" cabinet with 30" tall doors (no drawer), and put the plain cheap MW on the top shelf.

My sister built her kitchen with a shelf for a standard countertop microwave. It solves the issues of trying to find the exact size / trim kit , , , and doesn't take up valuable counter top space and they don't cost near as much as built in models.

We had a 27" wide built-in microwave at our last house. During the 16 years we lived there we had to replace it twice because they don't build things the way they used to. So you should probably make the space big enough for the standard size of built-in units, or relocate it. The built-in units are ridiculously more expensive, too.