I consider completing Dragon Slayer 1 my own litmus test of whether I should upgrade an account to members. On making it stick, that’s a difficult situation. There is always stuff to do in OSRS. Is all of it fun? Depends on what you like. I used to not mind Agility, now I hate it, but it’s useful to train. I’ve realized I love PvM and am working long-term towards end-game bosses, butt combat training gets boring for me after a while of the same thing, so I switch it up and train something else or play something else. Dual monitors helps a lot too for AFK skilling, having something to watch simultaneously is fun to me. Podcasts can fulfill a similar thing, or just vibing to music.
The biggest thing is knowing your limits to prevent boredom or burnout. It’s a gigantic game and a gigantic time sink if you want to make a lot of progress. If you get to mid game and can’t commit to longer grinds, l hate to say the game just might not be for you. But you can get a very good idea of the member experience with 30 days of membership, I think.