If you are going to use Mint 20 I'd compile everything natively on Mint 20 rather than using the Debian packages. The dev package can cause a issue or 2 with dependencies.
Even still I'd run with Mint 19.3 and build Linuxcnc 2.9 on that.
Actually you could use my Mint 19.3 ISO and build 2.9 on that. Or if there is a call I could easily make the 2.9 packages for Mint 19.x, but so far no one has requested that.
