Are there mobile desktops for Debian?

If you just want the DE and to configure everything else yourself, you can always just install phosh or sxmo on top of a debian/raspbian installation.

Mobian/Droidian are targeted for smartphone SoCs, so they would take a lot of tinkering to get runnin on an RPi

If you want a full OS that’s already configured to be a smartphone-like device, something like Glodroid may be your best bet. They’re an infrequent updater, the only reason I mention them is because they can target Broadcom devices (like RPis)

If you don’t mind getting away from building the hardware yourself, and just want a phone that you can run linux on, FairPhone/PinePhone/Librem 5 seem to be the way to go

Aside from that, afaict you’re in for designing your own device from extant components and then crowd-funding to pay for a factory line to assemble it for you (this is essentially what the PinePhone did)

realistically how feasible is it going to be to have something be completely free of it?

It’ll be feasible if we encourage a culture of not using it. It doesn’t have to be the main mode of development — and all the big names can keep their slop generators — but as long as there remains a demand for slop-less software, there will be people willing to make it happen.

There’s also the saying “don’t let the perfect be the enemy of the good”: besides the fact that I don’t trust slop-coders to put in any more effort into architecture/security/performance than they do actually learning the languages and writing the code, I also hate that they are willing to fund giant data centers that deplete local reservoirs and cause blackouts for small communities. In this case, I don’t care if it’s “no ai at all” or “no ai as much as is practical,” because both are still better than “full steam ahead.”