If you book on the QR website, it allows you to select the Classic fare that includes lounge access.(Yes this fare is on the QR site, no need to use a shady 3rd party one)

Depends on how much you value Al Safwa because on a different ticket I have been looking it, it adds $200 one way, just testing your route, it's just a bit over that, like $220 each way to bump it to Classic.

That's not worth it for a short-ish stopover, and I even question it on a long stopover. I haven't been to the Platinum lounge yet but reviews don't seem bad. I suspect that's going to be my drinking spot for a few hours in November.