Im so confused here ... you have a 15×15 room and cannot get an ac?
Sorry if i got facts wrong, alot of info thrown around.
I have same room i just built. 15×15 but has 8 lights. 24K btu minisplit and runs at 78° perfect without any of inlines cooling lights yyet. Room this size needs good ac and fans.
Myfriend has a 10×10 room with 9k port. Ac. 4 lights with hoods ducted in strait lines. He can hold temps so should be pretty easy for you with so much more space and an ac.
Port ac sucks but will work with only 2lights that are air cooled. Ducting should be same siZE and in strait line as can be and should not have issues is such big room.
I would get ac before co2 cause outside temp will dictate temp in room. Sart with basics before advanced. Meaning, temps,airflow,humidity,feeding. When you feel you cab control those then add some co2...added co2 doesnt benefit them unless they have more light anyways.