I don't fly QR anywhere near enough to be able to comment on flight loads and I suspect it depends on cabin class too.

As for lounges: you'd have access to AA Flagship Lounges at ORD, MIA, and LAX. At all three airports the AA FL is not in the terminal QR departs from, but if you're connecting there from DFW presumably on AA you should be well positioned-ish to access FL anyways; just save time to hike/bus to QR gate. LAX has the common OW J lounge at TBIT. PHL still does not have a FL, but QR specifically contracts to the BA Lounge there (which otherwise is weird at admitting based on regular OW rules.) QR contracts to the AF Lounge at BOS, presumably because the BA Lounge is closed during QR flight time? Or maybe just usual weird BA lounge shenanigans. The OW website is a good place to figure out which lounges will be available to you based on OW rules/cabin class/status/specific airline contracts: https://www.oneworld.com/airport-lounges

ORD requires a bus. Everything is connected airside at BOS, ATL, PHL, and LAX. ATL has the automated train. No idea how MIA works.