Where, specifically is the BGA? Crowns of the vegetative cones, apical meristems? Or on the sediment, below the gravel line or?
I have BGA below the gravel line closest to the window in many tank,s but it's well behaved and stays there. I scrape it every so often, maybe once a month, but it does not pester me, nor has for years.
So it's there, but does not pose any bloom issues.
You might deep gravel vac say 20% of the sediment each 3-4 days and replant the stems etc back after. This will clean the tank up nicely, and give you a chance to clean the filter, add purigen etc or whatever else you'd like to try.
If EM is not able to handle it post recovery/post treatment, then clean the tank good and dose well, check CO2 again. Black out works for getting rid of what is there, but only if the root issue is addresses while you do the blackout.
It's annoying, but is the easiest pest to get rid of and keep away.
Not that helps you any here
A good cleaning of the sediment and filter, trim, preening etc, every few months, maybe once 6-12 month cycles, not a bad idea. I do this with the ADA soils, I remove the dust/fines and leave the larger grains with the gravel vac.
O2 is easily able to move through the sediment and plant roots have access to it, if the plants are not piping enough down into the sediment(so the CO2 thing plays a role there likely also since good CO2 drives plant growth much more than any nutrient and more growth= more O2 to the sediment)
I do not think dosing is the issue nor urea. As long as the NO3 is say 5-10ppm or more, you should be fine.