Hey EnderUK, tank is looking good. The tree root is very nice.
I know I'd be shot but in your case I'd plant heavier with healthy plants that have grown submerged previously and increase the light, not decrease it.
I can't get a low tech to grow healthy plants and balance well with light less than 6hrs and that's on a tank next to a window otherwise 8hrs on all the rest. I can't tell you par readings but they need to be bright enough. I have the same size tank with 11W light on 6hrs a day next to a window getting direct sunlight, grows extremely well, no algae.
You need a combo of heavy rooters like a crypt or echinodorus(echinodorus parviflorus is lovely and small), fast growing plants(bacopa, ludwiga) and then the slow growers of your choice such as anubias, java fern or moss. Floaters are good when you start a tank to compensate for problematic growth below if the tank has issues at the start.
That's what I'd do. You started with low plant mass and low light in a soil substrate and this scenario is hard to make successful. I presume the tree root doesn't allow for heavy planting too.