Poor new growth means one thing to me, not enough co2. Low co2 could also be thought of as too much light, and to me, it sounds like a lot of light.
It's kinda like a game to me: How much light can you
use without getting algae and have gorgeous plants? The key is to keep up on co2 and nutrients.
Co2 is going to need to be re-evaluated every so often, doesn't matter if you had
it right once upon a time, it doesn't stay
right unless you keep it that way, i.e. plant mass increases, flow reduces, co2 tanks run out, drop checkers lie, surface ripple changes, leaks develop.......never overlook your co2 tubing when checking for leaks btw. You have to stay on top of it.
I use 2x54w t5ho on my 55g and I've had to raise it 2 ft. above the tank, even with high co2.
I would recommend raising your light fixture, max out co2, then incrementally lower light over the next few months and observe the point where problems occur, then raise it a bit from there and you should have good, manageable light.
Hanging the light has other benefits, as you increase the distance from the light to the tank, the light gets distributed more evenly among substrate, middle and upper portions of the tank. You get a more uniform par throughout the tank.
Also, it's a lot easier to do maintenance if you don't have to move the light.
Your other parameters look good and you're dosing EI, but surely you must secretly care for the lives of your fish right?