I know what you mean - at first it’s almost overload with info but it does get easier and you can apply common sense to most questions.
I have a midea portable unit for like 319$ on Amazon - 10000 BTU… it is expensive but I feel giving up half of my yield for one plant was well worth the trade off. Again common sense told me it’s gotta be more efficient buying a bigger one than I needed and not running it at 100% capacity all the time. It may come on for 5 minutes every hour during the day.
That small of light you won’t need much to cool. What’s your wattage? Seems a little low to get big flower… usually you’d want atleast 30 watts per sq and most shoot for 50 watts per sq… really it’s much more complicated getting into the par and ppfd but I’ll leave that alone as I do not have a grasp on it myself.
I would say leave the ac out for veg and mid way through flower then bring ac into the picture as you start cutting wattage back (if you have a dimmer) to tell the plant it’s time to finish.
In regards to flushing - just start cutting back the N as you increase the K. Not sure what you’re using but look at whatever your giving her and I bet you’ll have enough N in micro nutes anyways (check out Lucas formula for GH 3 part)