Yeah, that kind of soil bit me hard too. You're actually doing a lot better. Don't go chopping the leaf yet, even the worst one. When the leaf is mostly yellow or brown, then it can come off.
It's a tough call on your next step. Your new growth looks good, and that is more important. Rust dots are probably part of the nute/ph situation. I would watch closely for a day or 2. Track the progress. If it begins to deteriorate, be prepared to transplant. Read up on soil, transplanting. Your plant looks mostly healthy, it may well grow through the nute situation.
growone