Honestly, between economies of scale and supply side issues, you're just as well off buying a pre-built luminair. DIY isn't what it used to be.
HLG, Vipar Spectra, Medicgrow, hell even MarsHydro has way upped their game. Shoot for 40 watts per square foot in flower. The vast majority of...
Try raising your temperature to 84 degrees. Just to see if that fixes things.
Alternatively, a couple guys on here consistently run their grows well outside optimal vpd and they get great results. Maybe dial in physical environment until your plants are happy and let them run. Doesn't really...
Don't think box fans will have the static pressure to do much good in this application. Name brand seems overly expensive for sure, but some of that is the fans.