There's an increasing network of private lounges in the USA that belong to PP, such as The Club at ATL. IIRC other locations include BWI, JFK T4 (wingtips, near the DL gates), etc. There are also some new AmEx Centurion lounges (LGA, LAS, SFO, etc.) which are free for most unaffiliated AmEx Platinum and Centurion charge cards, while other AmEx (credit) cards can be used to purchase entry. AmEx Platinum and Centurion cards also get you entry into DL lounges when you have a same day flight on DL (not including codeshares). Note that not all PP memberships cover UA lounges, although if yours is based outside of the USA, it probably does.

You should explore the list of PP lounges as well as oneworld lounge locations. It might make sense to keep the basic version of PP for about $125 plus $30 per visit as insurance.

Some USA airports that have separate TSA security areas (usually separate terminals) for different airlines include JFK, LGA, BOS, LAX, SFO, DCA, DTW, TPA. At ORD, the international terminal (T5) is separate, while at MSP legacy carriers generally use T1 while LCCs are at the much smaller T2. IIRC SAN had two terminals plus a separate area for some commuter flights. Most but not all of PHL is connected airside and the same is true for PHX IIRC. STL, RDU, BDL, and MDW *might* relegate Southwest to a separate terminal but major carriers all seem to be at the same place. EWR is one building but had several distinct security areas when I was last there.