Android 8.0 or later
Decky Display
The tablet you already own, as a screen you can draw on
FreeThe client for Android. It shows the Mac's screen and sends your fingers, your pen and your keyboard back the other way, so it is a display you work on rather than one you only look at.
Touch that behaves like touch
Tap, drag, two-finger scroll and pinch, hold for a right-click. Focus a text field on the Mac and a badge appears on the caret to raise this tablet's keyboard.
Reach the Mac from the corner
A floating panel gives you Mission Control, Spotlight, the Apple menu, media keys and the switch between Extend, Mirror and Alone.
Cable or network, and the cable wins
Plug in for the steadiest picture and the least delay, with nothing to enable on the tablet. On WiFi it finds the Mac by itself.
It can be the source, too
A trusted tablet can send its own screen to the Mac, for the Mac to carry out to a Room.
Free, including for joining a Room. It asks for no permissions it does not need and there is no account.