I was hoping you'd join the party as your tank is quite deep as well.
Last time I lit a tank this deep (55g ~24") I used 4x 39W T-10s. Following along to see what equivalent LED fixtures are out there.
Ugh, found it again but seems to be a photobucket image so having trouble copying a link. For the OP try searching for "light intensity by height planted tank" and should get info on bulbs.
My tank is 2.5ft deep but runs injected CO2 either end and dry ferts dosing so a bit easier. I was told at the shops a single quad t5ho would do but talking to plant growers, they have run up to 2 x quad t5ho's which must pump out a lot of light.
I use a quad t5ho with red and white light bulbs (I don't think reflectors are great). And have added a decent LED strip light which is can be dimmed on white light bank and blue light bank (it's not great for plants but does the job).
I started off low on the LED and lifted it up until GSA showed up. Running white light bank 100% and blue light bank at 50% now and clean GSA off every few weeks. When I was using higher powered reef lights, was cleaning BBA off every week (shudder).
The test I found was would stem plants grow from substrate (good light), and then would plants grow sideways also at substrate (great light).
CO2 and soil (or a good substrate) also make a difference. So if plants continue to struggle and light should be reasonable, I'd go for CO2 injected. Liquid carbon imo (behind bunker) is nowhere close to injected CO2. Some thoughts
Last comment is that lowering the light from say a stand of several inches onto tank rim, I found gained about 5 to 10% par in testing.
Photobucket
https://www.google.com.au/search?q=...=5KbFWfPPG4PW8QWSqK24DQ#imgrc=4S2aMHAOTvhE6M: