be sure to not use greeter logins and disk encryption as the on-screen keyboards (OSK) don’t work outside the DE. also, issues with non-US keyboard layouts.

Greeters like phog or phrog are designed to work with touch and packaged in distributions.

not sure if you’ll be able to actually use the thing without an external keyboard/mouse; case in point, Gnome for the longest time didn’t allow touch to initiate dropdowns in the UI, kinda big deal.

No need for a keyboard nowadays (can say so at least for the DE I'm using). You can attach a keyboard though (and often a 2nd (external) display) if you want to turn it (temporarily) into a "desktop".

Phosh is used quiet a bit on tablets. E.g. Purism ships it e.g. for their Librem 11 and also Juno uses it. We have a phosh-tablet for that use case in Debian.