Some mountain bikers use footpaths very early in the morning, but it is, as you say, bad form.
It can also be bad riding, you will soon get fed up of sties, steps, gates and other obstructions.
Bridleways are mixed use, so no worries there, but they are often very rutted and potholed.
Your maps and apps will be a start, but ultimately it comes down to on the ground research.
It's still riding, but it can turn into work.
I've visited some areas several times, scoping about each time, before finding a ridable way through to the next part of a route.
Very satisfying when you do work out a route and add it to, in my case, my mental library.
A good tip is look out for bike tracks.
If you pitch up somewhere and there's a choice of ways forward, the chances are where the bike tracks are is where you want to go.
Other riders will have tried the alternatives and found them to be cycling dead ends.
Other riders are also an excellent source of route intelligence, so if I fall into conversation with another cyclist I will often ask 'which way did you come' or 'which way are you going'.