So it is time to start considering Lollipoping and setting her up for bloom as you said. That process may induce some stress which may reduce uptake but not likely. You know what you are doing now. So we don't want to change too much too fast but she is healthy and good to go.
So I would do some underbrush cleaning and start the transition as you still got some stretch to deal with. Then...well the more you grow the more you know...but it aint about how many tops you have so you may want to trim again post stretch we shale see but she is real bushy.
Your setup has inherent airflow issues so you wont want it too packed as we have talked about many times. Actually this one is way better than your first I think.
Perfection cannot be achieved.
Back to the PPM...I save it for last because all that other stuff matters right. So my answer always is do not follow a schedule until you have documented in this grow operation the process for this strain. The quality of the artificial environment matters. Once you have a god run down that worked well that was documented you can make a schedule.
Until then you want to read the res and do what the plant tells you.
It is dropping PPM so it is sucking up lots of stuff now meaning you are under feeding...but not by much I suspect.
So I would go up a smidgen. her physiology suggests closer to the 550 range...in a great setup that might run even at 600 growing fast...which sounds like she is doing now.
1.5 gallons a day is good uptake so the water does not need a lot of nutes...
I told you that you wouldn't believe how well clean roots work.
Nice job so far. Thanks for the update! Keep on relaxing and don't push her just listen and respond.