Pondering more directly my plans, I have downloaded all the suggestions y'all gave that I could find free pdf's of (OSE, Low-Fantasy Gaming, Dragonbane, Hyperborea, and several others). When I have some more dough, I'll make sure to send my cash to the ones I end up using the most, as I do like to support independent RPGs.
In addition to that, I already had Castles & Crusades (1e), Five Torches Deep, Knave, Deathbringer, ICRPG, Crown & Skull, the quickstart rules for Dungeon Crawl Classics, and of course, Shadowdark. I also still have ALL my books from older editions of Dungeons & Dragons (Moldvay/Cook B/X, Mentzer Basic, AD&D 1e, 2e, plus 3e, 3.5e, 4e, Essentials 4e); and of course 5e.
After reading all of this, I have come to the conclusion that the OSR community is probably a better bet for me than embracing 5.24e, switching over to something like Pathfinder, Tales of the Valiant, DC20; or picking up a new game like Daggerheart. As an aside, I've done some pondering and realize I really like my d20s, and prefer a "roll high" mechanic to "roll under" ones, which is a weird idiosyncrasy I've noticed in some games. I realize it's totally arbitrary, but I simply can't quite muster the same excitement for rolling a "1" as I can for a "natural 20." And despite loving d6 Star Wars, bad play experiences with Shadowrun and Hero totally soured me on dice pools.
So here's my plan: Use Shadowdark as a framework, because Kelsey's done an excellent job of writing a modern ruleset that (mostly) suits my tastes, and is compatible with a long history of adventures and existing old-school mods. For example, DMScotty's "Luck Dice" are already sort of part of it.
Shadowdark is also well-supported by the community, and I think it will be relatively easy for me to houserule anything I need to do to make the game work even better for the tastes of me and my players - to the extent it hasn't already been done by someone else.