I have been reading your thread from page 1 and is funny to see the transition you have passed 🙂
My tank is almost like yours, its a 60x30x30, high tech as well but with low light from day 1 (its 1 year old now)
What i can saw from my experience is that indeed ambient light was part of the issue since if you can see the tank with lights out, its a bad sign and algae take over by it,
Other part of the problem was the lily pipe...unless you had those lilies that were curved to the surface, you werent getting enough agitation and that rised big issues due to low O2 (your plants were actually consuming O2 at some part due to the ambient light, worsening everything)
Now that you have this spray bar (a cool sparybar i must say) stick with it, and always aim it to the surface to cause agitation but not ripple, that will cause a natural exchange of gases!
You had all the signs of low O2 distress by your fauna.
Now, using a DC inst as accurate as some think, even Tom Barr (the one i follow constantly to inform myself) states that.
To measure your CO2 with relative precision, use the CO2/KH table and a Ph Meter properlly calibrated (Ph meter pens are okay)
Your light is more than good, but if you want all the corners to grow, you have to place it upper and power it up more, this way all the tank will be more evenly lighted up (maybe reconsider the ceiling hanging 🙂 )
HM inst the best plant to foreground our little tanks and the sooner you replace it, be happier you will be, HC inst as light dependant has people say, its really CO2 dependant so the good flow provided by the spraybar is imperative as well as stable regulated CO2.
I even had a HC carpet on my low light tank, the only issue i had was lots of Brown fillamentous algae on the beginning and no fauna 1 year ago, but it was carpeting nicely.
All of what im saying is due to experience on my tank and reading a lot from Tom Barr and using Estimate Index.
Btw, notice that your spray bar is positioned such that the flow gets broken by the stones on the back of the tank, and with the low light, there is no change plants grow nicely there.
I hope i helped you, i like your setup and tank, dont loose your way, things wont be as hard as before 😉