I have come to think about having a QT/hospital tank this sort of way:
I have a similarly sized system as yours. There are seven fish in it, inexpensive ones so they're probably replaceable for ~$500. I'm a bit attached to at least some of them, especially the older ones (my oldest is a royal gramma I've had for 14 years). And they're living things, some of which were pulled out of the wild for my trivial enjoyment (and it is safe to assume that every wild caught fish in a tank implies the removal of 2 or 3 from the wild, counting the ones that died along the way).
At any rate, that's $500 I could stand to lose from the next fish I add. A decent QT setup can be had for less than $500. Also, I'll have that equipment for years (as opposed to a similarly priced pile of dead fish in the trash).
Also, if I bring some disease in, I'll simply have to watch them die without any good means of treating them -- that's demoralizing, and I need all the morale I can get. I could set up a hospital tank on the fly, but setting up a tank to hold and treat that many fish effectively is a lot harder than having a tank that can QT one or two fish at a time as I add them.
Keep in mind that QT isn't entirely about treating disease, contra the 'rapid dips' you mention. It is also importantly about observation (that's literally what quarantine is, though that word is misused in reefkeeping lately), and getting the fish adapted to new foods and water parameters (adjusting the salinity on a QT is a good gentle way to acclimate fish to reef tank salinity). It can also be a waiting period during which the keeper has a chance to back out of bad impulse purchases, which are a thing when buying from a shop that's a distance away; that six-line wrasse looked like a great choice at the shop and it was on sale and the guy at the shop talked it up, but during a month in QT the keeper has time to reconsider adding such a hellion to the DT.
Otherwise, consider buying only truly cleaned-up fish from a vendor who is in the business of cleaning up fish and has a reputation for doing so effectively.