Lots of good advice here. My own experience:
Advanced Navigation
Navigation you can learn from books and lots of practice in situations where you can bail out if things go wrong. As everyone has said, hone your map and compass skills and just use GPS to double-check every now and again as you go.
I strongly recommend The Ultimate Navigation Manual by Lyle Brotherton. He travels the world teaching navigation to Special Forces and SAR teams, so it’s laid out as a course with exercises. This is how the professionals do it, and if you master all the material in this book that’s relevant to your local terrain you’ll be a solid navigator.
You can practice skills such as estimating time and distance, taking bearings and back-bearings, planning attack points etc etc, even on safe trails or walks you know well. Then up the ante on safe ground by getting out in mist, rain and darkness – everything becomes much harder. Then try on new off-trail routes with easy bail-outs. Then go into challenging country, but with someone experienced – take the lead on navigation but have them check your work and point out any errors before they become a problem.
I’ve been navigating off trail for half a century and still practice these skills regularly – they are the foundation of safe travel in the backcountry and it’s never good to become complacent.
Walking on rough ground and snow
In contrast with navigation, I don’t think you can learn this independently. You need to hook up with experienced people who can teach you the skills and judgement involved, especially with avalanche risk and glacier travel. I mainly learned by joining a club – the experienced members are usually delighted to help out the newcomers. Failing that, you might need to go on courses, but I preferred learning from veteran members. I also learned a lot from watching the professional guides in the Western Alps, and buying them drinks in the huts so I could pick their brains. Read everything you can find. Get out with experienced people and ask lots of questions. Practice regularly. Repeat…
Good luck. Travelling safely off-trail is a challenge and a joy, and it sounds like you are going about it the right way.