Oh, yes. Exactly: float switch in sump to autotopoff switch to topoff pump in 32g ro/di reservoir---to reactor, via a check valve, [a BAS reactor with Hanna stirrer] and so on to the drip line, set above my skimmer intake in the sump.
I run a 54g reef with a 30g sump/refugium. The thing's been a tad cranky, but I think I've ironed out its problems. [The stirrer seemed to have been magnetizing the bottom plate of the reactor: a timer and giving it a rest has solved the problem.]
So far this totally supplies my alk/cal requirement for the 54g. I supplement mg now and again, and have wild coralline growth. It's saved me a lot of work. I was hand-dosing.
I use Mrs. Wages Pickling Lime [5.00 a can] for the medium.
On your 55 it ought to do great, maybe ditto for your 135. For your 180, I'd suspect a calcium reactor might serve you better.
Somebody in the diy section was making their own reactor out of a Rubbermaid bread storage box and some locline.
HTH