Hey...I believe I have at lest two solutions for you. Either is certainly is going to be better than White paint alone, and both ridiculously less than GOO. but it will probably cost approx. $250.00 USD for the paint, alone. Minimally. Goo would be well over $1000.00+
Being that you are in Spain, we'll have to work together to source some few components, and anything that is wholly unavailable in the EU I can probably arrange to have shipped affordable to you via USPS.
However I suppose time might be a factor for you, so relate any specific limitations, as well as what you deem to be your maximum budget, as well as to if the paint could be sprayed on in sections, or if rolling is the only option.
lastly, what is the minimum viewing distance? While most high gain, high contrast DIY Paint applications need spraying to avoid the reflective particles from being too apparent, if lumen output is widely dispersed and lower per sq. meter than average, often that is far less an issue.
You obviously need some boosting in the dynamics of the resolved colors as well as some assi9st in Black levels, so we must get you to a point where you surface will have at bare minimum 1.2 gain and be a slight silver/gray in shade.
PM me you specific location (incl. Postal Code ) so I can explore your local / EU availability options.
Sorry for the protracted delay in my response, I've been up in a Attic most of the previous 48 hours, laying out and labeling Fiber Optic Cables. The rest of the times in between I spend flat out recouping.