Consider an outline where you start with your protag--Fionn, whom you might consider renaming since this is secondary world stuff.
Start with his story, and see if you can dribble in all the other bits--dreams, prophecies, selections from ancient tomes--we're all familiar with the tricks of the trade. Consider making Fionn discover all this stuff for himself, so that the reader gets your world building in more measured doses.
Then write a first chapter. Put it and your other first chapter on SFF SYW. Get some feedback and give some critiques. SFF SYW taught me how to edit. It's not the sort of thing I could compress into a forum post. It's more of a learn-by-doing skill.
As far as marketability, it really depends on what you want to do. For instance, I wrote a biography of my husband's aunt, who really is a singular person. Born in 1921, she was a marine in WWII. Later, she taught art and traveled the world.
It sold its expected sixty copies or so and that's all, which was fine. I didn't write it to be a commercial success. I wrote as a gift to a woman I loved for her 90th birthday.
It's different with my fantasy books. Part of their worth comes from sharing it with an audience and seeing that audience grow. I want them to be commercially viable.
But there's a lot of wiggle room there. If sales were my only metric, I'd write pure romance--it's the biggest market. So that puts me somewhere in between writing only so that the work will exist and writing for an audience.
You'll need to decide where you fall on that spectrum and what you're willing to do to achieve these goals. And then you'll need to work on the skill sets you'll need to achieve them. Be patient with yourself. In my case this has taken years, and I still have some ground to travel.
Earthsea is an older book, but one that's held up well over the years. I personally admire Blackout and All Clear by Connie Willis. Or if you'd like to try a single volume, To Say Nothing of the Dog. Both of these stories rely on an incredible amount of world building (her protags time travel back to WWII). But it's woven in seamlessly throughout the narrative. Ms. Willis has won multiple Hugo and Nebula awards, so if it turns out that you like her work, it might be worth deconstructing to see if you can adapt her rhythms to your own writing.
Some of Pratchett's stuff has significant world building in it, but the tone is often humorous, and he doesn't really use chapters, so this might not be the best guide for you. Still, consider Mort, if you haven't already read it. One of the main characters is Death, so there's a fair amount of otherworldly world building.
I quite liked Diana Wynne Jones's Dark Lord of Derkholm, and that had a fair bit of second world theology worked into the narrative.
Can anyone but Tim Powers write like Tim Powers? Hard to say, but he mixes incredible amounts of world building into his work. It's worth reading, even if it turns out that (like me) you can't quite wrap your mind around the way he does it. His first novel, The Drawing of the Dark, is still my favorite.
Hope something here helps.