Hey mate good to hear its going well! I am using the same fertiliser as you. I add in about 100ppm for seedlings (additional to the ppm of the tap water), but for plants straight out of flask i don't actually fertilise them at all for a few months.
If you don't have something to measure the ppm with, try giving them a dose which is half of the 'weak' recommendation they have on the carton. I can't remember what that is but that's about what i use.
I only need to flush the pots out about once every 4-6 months. And the reservoir gets cleaned at the same time.
Because the reservoir runs out every week or two your basically replacing the water fairly regularly anyway so i wouldn't be overly concerned about the buildup of 'toxins' or pH change.
Just be careful and don't introduce any non-flasked plants into the same reservoir because its possible to spread virus that way. I say possible, because i haven't seen it happen yet, but people who have never tried it in their lives keep telling me its bound to happen (?). But definitely be mindful of what you put in the reservoir and don't cross-contaminate with dirty blades just to be sure.
P.S. good luck on the weekend! Your a braver man than I.