You're really looking at the google pixelbook, that has linux app support, pen support, there's no fully open source machine with all that. If you want something fully open you're limited to slower systems like the pi3. If the pi4 makes a major leap then the pi4 may be fast enough to use to build a tablet device around.
I’ve never seen this myself but what about using linux on an iPad like a thin client. There was a lot of whoopla recently about someone running the Mac from the iPad Pro in this way.