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.
Go and do something with it. Rock climbing knots are very useful for outdoor stuff. Start with basic rock climbing (sport) and get into trad style, then you’ll learn all types of knots.
Yes. Find something you enjoy that depends on knots. Perhaps a shibari friend.