If you are seeing new GSA, I would double the dose of PO4 (this is what I do up to 6ppm a week, after 6ppm I move up by 1ppm). Plants love PO4 and it doesn't cause algae. Just don't neglect your other nutrients too.
I've been as high as 10ppm and never saw any problems, I also didn't see any improvements so I dropped it back down to 8ppm.
These are the relative ranges of EI dosing. Tom Barr recommends 1/4 to 1/3 EI for non CO2 injected tanks:
NO3 - up to 20 - 30ppm
PO4 - up to 5 - 7ppm
K - up to 20 - 30ppm
Fe/traces - up to 0.5 - 1ppm
@1/4
NO3 5-7.5ppm
PO4 1.25-1.75ppm
K 5-7.5ppm
Fe/Traces .125-.25ppm
@1/3
NO3 6.6-9.9ppm
PO4 1.65-2.31ppm
K 6.6-9.9ppm
Fe/Trace .165-.33ppm
The only time I see a problem is when I push Nitrates too high. I find GDA starts showing up. It's usually up towards the light first then comes down the sides of the glass. I suspect it's too much light at a relative NO3 level. Once I see GDA, I drop NO3 a bit and I've found my sweet spot.
Hopefully my math is correct 😁
If you only dose the incoming water to 1ppm, the tank can't have more than 1ppm. I see your thinking, but I would try increasing your total to 1.5 or 2ppm. I would go a straight 2ppm if it was me. But for learning purposes, go in .5 increments if you choose to increase.