A nicely written piece Gareth, and welcome to the sport. I started on fat tyres in the ’90’s, went to road for a few years, and now enjoy my MTB again. I still ride my road bike sometimes, but unless the roads are quiet it’s no fun.
I’ll second what @leegee and @neilupnorth have said about getting some coaching on core skills. Whilst mountain biking is definitely safer from a motorised traffic standpoint, you will appreciate that the countryside is less predictable when it comes to surfacing. Being able to brake and corner safely on loose or slippery ground will go a long way towards keeping you upright. Being able to unweight and lift the front end of the bike confidently will add to the safety and fun factor too, enabling you to get up or down modest sized steps and obstacles, even if encountered unexpectedly.
Good luck and enjoy.