Interested in Language
This is a German expression, actually. I was wondering if it works well likewise with English.
"Until then I wish you all the best".
Yes, I'd put a comma after "then."
I'll see you in September. Until then, I wish you all the best.
I'm not a teacher, but I write for a living. Please don't ask me about 2nd conditionals, but I'm a safe bet for what reads well in (American) English.