Update- I disconnected everything but 3 items from the entire network to run a test. All that remained was our work computer (because it’s a PITA to reconnect), the firestick, and the PS4. The computer and PS4 are hardwired. Speed tests on all 3 of those items showed around 150MBs. I contacted AT&T, and they ran some tests. Supposedly the modem/ router is faulty (a faulty dynode?) and they are setting an upgraded replacement (BGW320). Judging from the AT&T community forums, there’s a chance this will improve the speeds, but will not likely resolve the issue entirely (looking at 300-400, hopefully). Once I get the new unit installed and tested, I will update again. If it does’t work, I will probably hardwire what I can and then set up a new access point.
Final update- Got the new gateway hardware and it fixed the issue. Initial scans provide for 600-700 MBs on wifi. Thank you everyone for your help/support!