Thanks for your advice! I think you're spot on about the earlygame. I'm going to skip prepping Flame Magnet in favor of Extra Mana Boosters (maybe Extra Health Boosters???), because even with no fireball and a useless codex, Sorcerer + any spell can stomp through the first few levels via Essence Transit, and heavy fireballs aren't great for farming Mystera piety. Oof, though, that last-tile BURNDAYRAZ sounds awful and is exactly why I took Flame Magnet, pretty sure that'd be a reset.
Picking up potions is definitely a mistake, I really need to break the habit of double-clicking every potion I see. In that run I instantly regretted it. I should also be more careful with piety sparkles, I try to avoid picking up too many of them needlessly (for exactly the reason you said!), but it's not always at the front of my mind and I get careless with it. I also tend to rank "avoid picking up piety sparkle" a lower priority than "pick up thing on ground that piety sparkle is on", which is not always correct.
Sounds like a sweet run! I don't think you needed Platemail there, Witchalok and Fire Heart are the other two great defensive items and, despite being weaker than Platemail early on, they scale really well to tower floors 7-9. (It would have helped against Horatio though!) Not finding any non-Binlor ways of removing resistances is a pain, but at least if you've taken Mystic Balance ENDISWAL is almost as good as BYSSEPS for getting that done (it costs slightly more mana and slightly less health).
I'd love to see a video of your run if you have it! The Extra Attack Booster prep makes it sound like, even though we both went Gnome Sorcerer of Mystera with Avatar's Codex, we still had
very different playstyles during the run.
Now, if you think my neglecting attack boosters is bad,
watch me desecrate Tikki Tooki at level 1 as a Codex Sorcerer in Curious VGT 
...even though I have my reasons, that desecration is a bad move and I get super punished for it down the line, and I certainly don't end up winning. But I do manage to which is
way further than I ever expected I'd be able to get without infinite shenanigans. Even though it's a loss, I feel like it's still worth posting because, hey, it's Curious VGT, even reaching Horatio is an accomplishment?
Ironically, here I got in trouble by
not picking up potions on sight: on the golem-spawner floor, the spawns blocked me from getting to the mana potions I needed downstairs.
I'm going to try some more Sorcerer Codex Curious VGT runs, I want to see how far I can push this!
EDIT: Got even further this time!