Honestly if you’re going to live in Scotland and MTB , you’re going to have to drive, so much of the good riding is inaccessible using public transport, or public transport is just impractical.
There are great rides from a lot of the cities above, but you will be quite limited.
My choice would be Perth, good riding from city centre, easy rail link to Dunkeld, Aviemore and down to Stirling.
You will love it up here though.