There are many alternatives to lighting .Just a thought...
I’m reading along trying to learn, too...
So, I won’t attempt any real advice. I’m putting this thought out there to see if vetteguy agrees.
If his budget is $300, I know the kessil lights are pretty expensive.... would this work to buy time? Buy a cheap T5 grow light and purchase some good Ati/German bulbs. Until you could upgrade to a Kessil/tuna blue?
a cheap t5 is perfect and will grow the corals they want to keep
Over a 20 gal , a couple 24” bulbs will be sufficient .
The only other expense would be bulbs and bulb replacement