Do you have a built-in/drywalled pantry elsewhere? If so, then consider putting the MW there - but, be sure the pantry has very good venting - perhaps the door could have louvered openings or have both a warm air and cold air return in the pantry. Or, no door at all on the pantry?

Steam is inevitable when MWing food since water is key to MWing. Is there any other shelf you can place it where it's away from food so the steam and odors do not affect the quality of your food? Over a wall oven? In an upper cabinet elsewhere? In another room? Finished basement?

Unless you live in a year-round warm and relatively dry climate, I don't think I'd put the MW in the garage. (Ditto, btw, for any machine/appliance not built for use outside.)

Yes, I do live in a year round warm/dry climate (we run heat less than 10 days per year and AC about 30 days per year) and garage is kept clean. The pantry is divided into two sections vertically and the microwave is planned to sit on the bottom shelf of the top section. Microwave would only be operated when the pantry door is open (no kids in house). Most food will be in the lower section so I'm probably more worried about damaging the cabinet than damaging the food in the pantry. Unfortunately the house only has one small coat closet on the ground floor so if the microwave were to go there the vacuum which is used much more frequently would need to go into the garage. We decided to try a CSO above the oven so maybe if that proves unreliable we'll replace it with a speed oven/microwave long term. Thanks for the reply -- I'll try a few things.

I think if you are only using it for a minute 5x a month you can do whatever you want.

I just checked my countertop one and the air coming out the back is cool air vs warm

This manual mentions nothing about venting. Neither does the one I currently have.

https://www.ajmadison.com/ajmadison/itemdocs/JEM3072_Manual.pdf

Interesting, do you know the brand and model if you've bought it recently? The main reason I noticed this issue is because my temporary kitchen is in the master bathroom and I noticed that the mirror fogged up behind the unit.

Interesting Stan! Mine is a cheap Oster I picked up for my remodel. Ours is in a regular room and I haven't noticed any steam or warm air. Maybe move yours to a regular room and see?

Here is where ours in. It's a tight corner but still has air flow around it. The only clearance I've read about is with the plug

Another thing, lots of countertop ones come with an optional trim kit that can be installed into a cabinet. I didn't see anything there about venting either.

I had a microwave in my pantry for years without a problem or damage. Of course there are smells from cooking, but they dissipate quickly. You should be fine, especially if you use the MW so infrequently. BTW, if you have an induction cooktop, try using it for cooking oatmeal. Put it on a low setting (set the hob timer if you have one) and you don't even have to stir it til it's done!