I have a FB friend who makes tomato powder. I'll ask her how she does it.
I sent the friend a message on FB Messenger. Here it is.
Me: Hi, you once told me about dehydrating tomatoes and making tomato powder. Someone on the cooking site I frequent can't get his tomatoes dry enough to be crsipy. Didn't you make it into a puree before drying it? Any tips would be appreciated.
Friend: My favorite is the skins. Those crisp up really nice. For the others I just did slices or whatever usually, which can be a pain. BUT. I have done the puree. I did it on fruit roll sheets and did it kinda thin, but not so thin you can't get it off later. It's pretty important to break it all up when it's mostly dry, flip and rearrange it all and dry it even more. It helped to put it back on just the screen rather than back on the fruit leather sheets. It took a lot of drying. If it's really humid there it might just not be able to. I've also done the puree, broken it up into large pieces, conditioned it, and THEN put it back in on the screens.
Me: Conditioned it? And how thick were your tomato slices? How does someone know that it is dry enough?
Friend: I really don't remember how thick they were. It has been more than a couple of years. And it also kinda depends on the tomato anyway? Some have a lot more water than others, ya know?
Conditioning is basically putting it all in a closed jar for a few days and shaking it all up. If there are any moist bits, they'll dry out evenly into the others and not leave a damp bit where it can get moldy. If I wanna make a powder I'll do that to a thing (like the pureed garlic I like to make garlic powder out of) and then dehydrate it again. It's important to get all the moisture out for a powder or it starts to get like musty smelling.
it's a bit fiddly and I do a lot of it just by feel/look, ya know? But those are the best tips I use for this in particular.
And here's the last bit, that I forgot to include:
Friend: I've also been known to dehydrate the crap out of cherry tomatoes and then freeze or fridge it. Later I'll take some out, dehydrate it some more again and then make tiny batches of powder for when I need it.