You can't open this version of dosbox in full screen as it's SDL 1.2 and that's not fully supported on the Pyra yet. I have dosbox-staging ported which is SDL2 and I'll upload to the repo tomorrow.
Usually it's ALT+Enter, but I suspect it's still the issue of needing SDL1.2 optimized for the Pyra as it seems to be creating a portrait sized frame-buffer instead of a landscape
It's not surprising that skyroads doesn't work, it's from way back when before the Pyra was even released and nothing stays still especially before release. Supaplex depends on dosbox-staging, I'd expect it to produce a messge if you don't have that installed already, but I'm not intimately familiar with it myself.