For the doors- use pocket doors or barn-style doors to maximize floor space
Framing- try to find structural foam panels if you can, used is just fine. My ex’s parents built a superinsulated chicken coop out of those in a 8” thickness and the birds did just fine out there through 0F winters and kept laying with just a pair of heat bulbs going, which is pretty unusual. Failing finding that at a good price I’d sheathe the walls internally with garage door foam paneling, it already has a shiny alumnized face that is waterproof and pretty reflective. Framing could be pallets if you can find a bunch of clean and matchy ones for free, a double stack on the long way with the rails acting as vertical studs is just the right height for a garage, just sister them together with more scrap wood. Pile some fiberglass batting on the roof when you’re done for more insulation.
Ventilation- if it’s in an unheated garage you probably want to run it recirculating internally with scheduled exchanges from the garage air, which will be then need to be heated. If you could pull your fresh air, if possible, from the foundation or cellar of your house instead of the garage? That air will be way warmer and moister than raw winter garage air and save you on heating costs. Exhaust could just be through a window register or one in the wall, or really whatever.
I agree with @GCBudz that the design looks really tight to work in
I would read a bunch of @ReikoX ’s posts about his grow room build and how it works, it seems like that’s what you’re going for in a simpler form and he’s got it dialed and documented. This isn’t his current log but it starts with a good overview of the system: