I think getting a couple of tents is a good suggestion. A 2’x4’ tent for veg and a 4’x4’ for flower would only set you back a couple hundred dollars. Far cheaper than lumber, sheetrock, screws, mud, tape, paint, flowing, trim, etc. Easier and quicker to get setup, and can be taken down and hidden if needed.
Trust me, I’m mudding and taping a 4’x6’ flower and 4’x4’ veg room now. It’s taken me about 5 months so far, though the lockdown has slowed progress for two months.