A 1.5xBW front squat for 6+ reps is fairly advanced. Many other coaches would say so as well.
With that in mind, honestly I’d keep your training the way it is as long as you’re seeing muscle gains. PRs will happen naturally as you go along. The 335 seemed so much heavier because, well, it was heavier. Hit a hard 6-rep set on basically any lift then increase the weight 20 pounds, and you’ll be down around 2-4 reps.
You could think about spending 4-8 weeks going strictly strength-focused and then come back to the hypertrophy work, but I wouldn’t bother trying to tweak your current plan if it’s productive otherwise.
Technically, your “quads day” is “legs day #1” and your “hamstrings day” is “leg day #2”, because you’re not just hitting quads or hamstrings on either day.
With the exercises you’re doing, your upper back is getting worked on leg day #1 with the front squats and clean-grip reverse lunges; on back day with snatch-grip deads and the rowing work; on shoulder day with the SGHPs and face pulls; on leg day #2 with the Zerchers; and it’s getting some work during the farmer’s walks and sled pulling.
So I don’t think a weak upper back is holding back your front squat. Direct ab training is almost never a bad idea. There are tons of articles explaining how to implement ab work. Anything that has some flexion work and some anti-“whatever” work is a good place to start. Bang-for-buck, I like Pallof press iso-holds (basically this at a cable station, only holding the “out” position for a 5-10 count), 1-arm planks, and hanging knee raises.
Even just front squat overloads - loading 20ish% on the bar from your heaviest set, unracking it and standing still for a 10-15 count - can build core stability.