A couple weeks ago I was testing the git version of Btrfs Assistant (here) on Sway and I had narrowed it down to these environment variables:
export XDG_RUNTIME_DIR=/run/user/1000
export QT_QPA_PLATFORMTHEME=qt5ct
export QT_QPA_PLATFORM=wayland
export WAYLAND_DISPLAY=wayland-1
By adding these lines to /usr/bin/btrfs-assistant before the call to btrfs-assistant-bin it would launch from the command line with sudo.
I noticed it will run without export QT_QPA_PLATFORMTHEME=qt5ct , but it will not express the theming (it runs as a plain grey windowed app).
I haven't tested it since then, so I'm not sure if this method holds up with the changes that have been added to Btrfs Assistant in the past couple weeks.