proper mixing is 90% of the struggle of macarons. you want to fold the meringue and almond flour/10x mix till it becomes shiny again, like your meringue should be. I fold 3 or 4 times after everything is combined and check and adjust the consistency. Some people say it should flow like lava...great. I like to mix till it ribbons off the spatula. not big fluffy globs dripping off, but a 'flat' batter when it comes off the spatula.
do not overmix the meringue. I shoot for a 'bec d'oiseau', or 'birds beak'
If you whip too much air into the meringue, you need to get it back out. That results in over mixing. Tapping the trays on the countertop helps to eliminate any extra air inside the shells before they dry. dried shells that are ready for the oven should not stick to your fingers, should not be tacky.
It is important to note that I am at high altitude as well. I bake around 350 to get good oven spring in a larger commercial oven. 5 minutes, rotate, 5 more and pull them. at home i will go down to 300, and bake 8, turn then bake an additional 3.