It seems from posts that people in your position ended up finding that
adding some extra K resulted in dramatic improvement. I think this is why
the popular PMDD formula has and continues to advocate adding K+ as Cl or
KSO4.

KNO3 would then be the "optional" chemical if there's a low fish load or
low NO3 in tap water.

Even if a plant has more, needs more K relative to N and other
variations are considered, 4.3x as much is such a large factor that K+
will not become limiting in most planted tanks using KNO3.