I use a more dilute fertiliser solution, not exactly EI. I just don't like buying water with some salts in it. I don't really find mixing salts complicated at all, I'm accurate with my measurements and it has never gone wrong. I do it every water change after all (adding minerals to RO water).Sorry to see you are still having a rough time with this tank.
I strongly believe that before you do anything else, get some floating plants in, see how they perform.
As others have said, though I know you have struggled with it, next get in tonnes of cheap fast growing stems. Hygrophila polysperma or even just plain cheap cambomba aquatica. You want a dense fast growing jungle to get things going.
I would be inclined, given the low light and non CO2, not to mess with EI salts. Get an all in one liquid food that is easy to dose. It's easier for you, and helps to rule out another bunch of variables.
Keep it as simple as possible, and try to change only 1 thing at a time. Don't underestimate the power of consistency, certainly at the start of a tank or recovery, you need to nail the consistency.
I certainly do agree that being consistent will reward you with good plant health. It's just easy to lose motivation when the tank is doing badly to be quite honest.