Have you looked at the geometry of the bike vs a gravel bike designed that way from the start? They are different, enough that your going to need to make compromise to get the bike to work/fit right.

If your MTB only has disc brakes (which is almost any under 15 years old) getting disc brake, esp hydraulic will be expensive. You say your not concerned about brake compatibility, this could be a project killer, and need to be considered from the start.

Not trying to put you off, but having done what your planning on a rim brake MTB earlier in the year, it's not an easy conversion to do, purely to get the bike to be comfortable.

Your planned gear ratio looks a bit limiting, without knowing the front, 11-42 on a single chainring may be limiting. if you want to get anywhere at any sort of speed.