Large water changes are not always needed unless you have a ton of waste to remove and can actually be bad for your tank in some cases. Otherwise, you may only need to change 15-20%.
The Watts Per Gallon rule is out-of-date and doesn't really apply to anything beyond T8, so it's really best to ignore it. PAR is what matters and there's plenty of data about it in the lighting forum (check the sticky). Though, it would be a good idea to get better lighting.
Bacopa monnieri (Moneywort) can feed heavily from the water column but also grows quickly with just root tabs. May be a good idea to get some to put into your substrate. Dwarf Sag and your other plants will also benefit.