Insulating that little room as a “cooler” for your grow tent should help keep things stable, and adding that small heater for winter should do the trick.
Since crawl spaces can get stuffy, good ventilation will make a difference, especially for air exchange and humidity control. I’ve heard of others using a similar setup and even reinforcing it with some extra intake and exhaust to manage airflow better. If you ever decide to go bigger or need some ideas for a future project, places like
topgradeenterprises.com offer supplies that might fit into a DIY setup like yours.