I’ll wait for a response from the Sway team on GitHub. It would be cool if DietPi could automatically set up Wayland by default.
That is not on any ToDo at the moment, so nothing worth to wait for, unless you start implementing it 
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Here is a way to check which packages were installed from the RPi repository instead of Debian, and can hence contain changes, some of which make them incompatible with other desktop/window manager packages from Debian:
dpkg -l | grep '\+rp[it]'
So if some library or other Wayland or GL related package does not behave as expected, it could be tried to “downgrade” to the one from Debian.
EDIT: Packages from wayland source are definitely affected: Index of /debian/pool/main/w/wayland
Was your system distro upgraded from Bookworm to Trixie, or a fresh Trixie image?
EDIT: And the RPi 5 kernel is a 16k page size one, which is otherwise uncommon for ARM kernel builds. One result is that it does not support 32-bit code. Might or might not be relevant, just collecting some info which makes Raspberry Pi 5 with RPi Ltd kernel different than plain Debian.