Hi Jaap, You could try changing your CO2 diffuser to the outflow but I’m starting to think natural systems need stability over all else. My plants are really fussy about changing things too often and too quickly.
The light question is a really tricky one. My tank has quite a few deeply shaded areas as well as very bright spots - up near the surface the light must be stupidly high but the plants are as healthy as I could hope for. The moss, HC and UG are constantly going from submerged to immersed as the water level drops through evaporation with no problems. Staurogyne and Hygrophila araguaia grow in the shaded low flow areas and the high light, high flow areas. I’m starting to question if there’s a ‘magic formula’ at all! I’m starting to think that as long as you keep the tank as healthy as possible (lots of good clean water changes, high plant mass, clean filters, constantly picking out dead leaf material and a clean, film free surface to maximise gas exchange) the plants will adapt and thrive in a very wide spectrum of CO2 and light levels. The only real difference I can see between our tanks is the amount of plant mass - you could always try a ton of cheap plants already adapted to a submerged life?