I think I'm finally narrowing down the search for a light after doing a lot more research today.
I'm still considering something DIY with floodlights, but I'm leaning toward a premade setup that would be more sure to work well.
The main contenders are the Fluval Fresh and Plant 3.0 ($160, 46W), the Finnex Planted+ 24/7 V2 ($128, 35.8W)(there's also a Planted+24/7 HLC; not sure what the difference is), and the Current USA Satellite Plus Pro ($242, 50W). There's also the Finnex Monster Ray and Vivid+, but it seems like neither of those is meant to be the main light.
It seems like the Planted+ 24/7 gets the most recommendations of the three, but it's quite short of the Satellite and 3.0 in terms of wattage.
I couldn't find PAR data for the Fluval 3.0, but its prior version was supposed to have similar (if slightly higher) PAR output.
It looks like the Current USA leads the pack for PAR.
At 12" deep, Fluval reports 112 μMol/m^2/s (I think that's the right unit), Finnex 88, and Current USA 155.
Down at 18", Fluval is 66, Finnex 53, and Current USA 98.
At both depths, the Fluval is about 80% the Finnex, and the Current USA is about 140% the Finnex. Then again, the Current USA costs 150% what the Finnex does. Toss-up between Fluval and Current USA.
All three models can be programmed for a day/night cycle with ramp-up and ramp-down instead of a harsh on/off, although the Fluval has the advantage of being controlled via app instead of with a clunky IR remote.
Because of this and its sleek look, I think I'm leaning toward the Fluval. However, my concern is how low it's mounted. Even on my 20 tall, which is 12" wide, lights can't be mounted too low. On a 40 breeder I'm afraid a low-profile light will leave large dark spots on the front and back top edges. Do you guys have any suggestions? All I can find is this hanging kit