Your EI dosing is correct, you don't need a "good substrate" (what ever that is) as most aquatic plants can take nutrients from the water column (else how would people get such wonderful tanks with sand ??) and 55UK gallons -> 66US gallons -> 1.3W/US gallon, so light not excessively high.
The following facts all point to one cause:-
To not beat around the bush, getting straight to the point, you CO2 levels are crap
as indicated by the plants. You have CO2 level issues, the plants are telling (if not shouting) out this. Changing to T5's will just make a bad situation worse, as you have found out.
You don't state if you have a drop checker or pH pen to ascertain CO2 levels, but in my experience (unless you are a very very very very experienced DIY CO2 person) there is no point in them as they will all read extremely poor CO2 levels. DIY CO2 is very fickle and very hard to get any decent successful level of CO2, even enough CO2 for T8 light levels.
Remember 99.999% of aquatic plant issues (seen here on UKAPPS), even with T8 lights, is poor CO2 levels. All this Ca:Mg ratios is a distraction from the what the plants are saying.
So way forward:
- Pressurised CO2 to get proper measureable CO2 levels. I have pressurised CO2 cylinders at work, hanging on the wall (how dangerous might fall off), they are call fire extinguishers. No one has died yet.
- Use liquid carbon.
- Halve or even quarter the light levels and hope that fish poo and rotting food provides enough carbon.