Your algae appears to be staghorn, not BBA
@Tom Raffield
Usually associated with the buildup of organic waste and crud. Cleaning the filter a bit will likely have helped, maybe increasing the frequency of filter maintenance is a good move. You could also remove some more of the oldest plant leaves and generally do some tidying on the bottom of the tank. Sometimes you can suck up some mulm without having to disturb the substrate much, you would be surprised how much can build up in nooks and crannies.
Other things to note is that some of the leaves on the hygrophila are now showing quite strong veining - is this a deficiency and if so, of what?
Some of the leaves appear to be deficient in Magnesium, which makes sense when you say fertilization is infrequent and you dont change much of the water.
Unless you are adding Mg seperately in some way, odds are your plants could use some more of this on a regular basis. Normally chasing down individual deficiencies is a bit of a wild goose chase, missing the overall bigger picture. But Mg is one of those which are easier to identify and makes a slightly bigger difference in my opinion.
I dont think this is the sole cause of the algae, but it is something that can contribute to healthier robust plants.
You can start with just some Magnesium on a regular basis and see how the plants respond. Since your tank seems to be going steadily it is possible that the macros and micros you are adding (although infrequently), is enough to keep the plants going.
What is your TDS currently at? I would monitor this value, aiming to feed the plants a bit more without causing large increases in TDS.
The TDS tells us how much "stuff" we have in the water, but not precisely what the "stuff" consists of.
If youre not changing much water I would want to avoid feeding the plants much more than they can eat, so you definitely dont have to go full EI.
A little is a lot better than nothing for the plants.
Edit: If you could squeeze in a few more water changes I think that would be good against the staghorn.
Other than this I think plant health overall looks very nice