Well, it's going to tough to make any predictive estimation with K+ with such low frequency water changes.

You can buy a relatively inaccurate turbitric Lamotte or Hach Test kit, but it'll give you about a + or - range of 10ppm in that 10-20ppm range

So it's not much use.

Another method is to dose based on the N uptake, but that assumes you know what you add there fairly well also. Fish waste often gets consumed as fast it's produced in aquariums. Those feces drop intop the substrate and very slowly release the NH4/NO3. Generally at ranges that exceed the test kits you might have.
Same with the PO4 from fish waste..
you can try and weigh and measure the fish food before you add it, get an idea of the N/P/K content.

Given your constraints, there's not much you are going to be able to do here.
Just eye ball as best you can.

Note, when you start adding more K+, the NO3/PO4 and CO2 demand will also go up as well.

That can lead to other issues more serious than low K+.

Just warning you.

I'd add 1/4 teaspoon K2SO4, 2x a week, buy a Lamotte test kit, make sure the K+ does not get much over 40-50ppm.

Realize that other nutrients will be uptaken faster now that this is not as limiting.

Regards,
Tom Barr