Default on-screen keyboard and Onboard show up at the same time on Ubuntu 20.04.3 LTS
Solution 1:
There are two workarounds. One is, as sudodus explained, to install xubuntu desktop and switch session at log in screen to Xubuntu. Second workaround is to install gnome extension block caribou. That way you don't have to switch to Xubuntu if you want to keep using Gnome desktop as I do.
Block caribou extension: https://extensions.gnome.org/extension/1326/block-caribou/