I'm in the very blessed position of having all 3. In terms of "most used", we rate them:
I (politely) disagree that jams, sauces etc can be as easily done without the Thermomix. I can't just dump everything into a pot, set it to go and come back in a while and have it done, and I don't have the inclination to stand over a stove and stir and what not. I have surprised myself with what I can now make myself with the Thermomix, that I would never have tried before. But OP, you are right, that the recipes do tend to focus on sauces and stews and all. That's not to say that you can't do other things with it, but those are the things where you really notice how hands off the Thermomix method of cooking is (that, to me, is a huge advantage of the machine).
The Airfryer is not something I would ever have bought myself but it was given to me for Christmas and I use it a lot. At least 4-5 times a week. I mostly use it as a mini oven - saves me turning the oven on just to roast a tray of veggies then having to watch like a hawk that my toddler doesn't wander over to it. I'm loving that we can make or own chips and snacks don't need to have much oil in them as well.
Poor old KitchenAid has taken a bit of a back seat to Thermomix, but it still stands proudly on my kitchen bench (mostly because I painted a feature wall in my kitchen to match it). I actually use it more for the attachments that you can get for it - the ice cream churner, the pasta maker and the juicer are the most used. I also prefer the consistency of egg whites and cream whipped in the KitchenAid, over the Thermomix. There are definitely some dishes where I use both the Thermomix and KitchenAid - such as ice cream where I will prepare the creme Anglaise in the Thermomix but churn it in the KitchenAid.
OP, have a think about what you really want to do with the appliances. If it's to branch into things like making your own sauces, dips, jams etc as well as basic meal prep, then consider a Thermomix.
The KitchenAid itself can't cook, but the attachments it comes with are pretty nifty - perhaps if you go down the KitchenAid + AirFryer line, which is much cheaper, you could pick yourself some attachments for it?
Good luck with the decision making! (It's a very pleasant tough decision!)