This is similar to my situation as I save for a quality CO2 system. Im dosing excel everyday and dry ferts once per week. 40% WC every 6 days due to bio load from fish poo. Seems to be going okay ATM.
Q: Would It be better to dose NPK over several days or just once when I do the WC?
Seems fine to me.
With CO2, 2x a week seems to be a min with decent results over a wide range of hobbyists using the gas.
Excel? Not so much.
Uptake is reduced from 10-20x down to 3-4x.
So demand and the amount needed to prevent limitations is also correspondingly lowered.