I believe knot-tying would be a great, life-long skill to have, especially being able to know which knots work best for different situations, like when the rope is thick and not very flexible or bendable.
I have lots of nylon rope and fishing line of various diameters, but don’t know where to begin and/or the best way to get started. My goal is to make this a long-term hobby and spend 1-2 hours per week improving my knot-tying knowledge, skill, and speed.
Find an old Boy Scout manual. It has most of the relevant knots/lashings and their intended use, along with stuff like first aid training and a bit of wilderness survival. It should be easy enough to not be overwhelming and give you an idea if you want more advanced knowledge on a topic.
Although for some of the more complicated knots like the bowline and sheep shank/bend might be easier to learn from a video.
But do keep in mind that there’s a chance the first aid info is outdated. Standards in wound care and even CPR have changed a lot since I was in the scouts.