Honestly man, I know its ur first grow n all, as u get a couple more u will figure out the trucks and what not I need to know for your particular set up.. one option you do have, is making some sort of access hole down thru ur media, I've seen ppl use a piece of pvc,or u can abort the entire fresh swap out Rez idea, go ahead and scrog it, as long as ur able to lift it just enough to see what u have in there, you probably could just top off every week instead of doing an entire change, it's not "ideal" bit I can tell you with absolute certainty you can go the entire flowering cycle without actually swapping your Rez. I had to do it once. I just let my plant drink waaaaaaay more then normal before I topped it off to the point where there was only about 1/4-1/2" of somution left.
Like I said it's not ideal, but if u have faith I. Your nite mixing abilities u can get it down pretty easy. Doesn't the root spa have tube for a solution sight level? I thought they did, u could pretty much do an entire Rez swalt just draining from that and then on ur refill just go about it like ur topfeeding.