In a non CO2 method, I can estimate the growth rates/uptake deamnd and add just a little less than that.

None will build up.
Even if it does, simply do not dose for 2-3 weeks will purge any left overs.
You will see the plant growth slow and various other changes occur.
As growth is slow, so is the response time to problems, deficiencies and their appearance. So you can respond by adding a bit more ferts as needed.

We are not dosing many nutrients either, we are still estimating, but on the lower end, preventing things from running out for too long.....for this slow growth rate.

The rate is much slower so we can use and get away with management methods would could not using EI etc. Likewise, not doing any water changes and expecting the fish to supply everything to a CO2 enriched higher light tank would be a lot more trouble.

They both work based on the rate of plant growth as does the dosing........... and the methods used to achieve that.

A non CO2 tank grows about 5-10X slower depending on the species.
Fish will help/do it all for nutrients but topping off a little KNO3/GH/KH2PO4/Trace every so often and then not dosing all week etc will help, espeically with pickier species. The same is true for lean CO2 enriched tanks where folks try and run things supper lean without much source of nutrients.
In the non CO2 tanks, the plants have all week(or more) to use them up.

We just a small amount of a balanced nutrient mix every so often to keep them all semi happy (happier than with only fish waste and sediment).

Fish food does not have a lot of K+. That can limit things in such tanks and there really are no good source in the sediment for K+.

Regards,
Tom Barr