Wow, that does indeed appear to be
major crappiness, sorry to say mate.

Although, that red lily seems to be doing quite OK.
I'm not familiar with Lush Max, and I guess you are buying it because it's cheap, which is great, but I wish you would buy your ferts from any one of our sponsors to help support the web site.
In any case, a 200L tank is a fair size, and using Easycarbo won't be cheap. It needs to be added every day, prior to turning the lights on (perhaps 15 minutes or so prior). I would start by using 2X the bottle recommended amount.
Those plants are suffering a bad case of Black Brush Algae (BBA) which is always a sign of poor CO2.
The filamentous algae on that Ludwigia looking plant and the blackening of that (fern) looking plant on the right also corroborate a CO2 issue.
I would definitely not even think about water "parameters". That will just get you deeper into a quagmire and you are deep enough as it is, but you need to do massive (50% or more) water changes at least once a week.
Start with that and see how it goes. I noted that you mentioned having used Easycarbo and how it seems to have damaged your plants but others have reported good success with those particular plants so I'm not sure whether they melted because you were not using it often enough or not. Very difficult to analyze.
Cheers,