EPR is the amount pressure decreases during exhalation. If your pressure is set at 7 cm (IPAP) and EPR is 3 cm then your exhalation pressure (EPAP) will be 4 cm. The machine will maintain 4 cm and then when you inhale it will increase the pressure to IPAP, then once you start exhaling it will drop the pressure back down to EPAP and so on. You can visualize this by looking at your mask pressure graph in OSCAR. Although you are correct that a higher EPR will drop your average pressure it does so by lowering EPAP which can cause apnea issues.
EPAP is what helps hold an airway open preventing collapse. EPR makes it easier to both inhale and exhale because the changing pressure helps air flow. This is why higher EPR usually helps overcome flow limitations.
If you lower EPR then you can also lower IPAP by the same amount while maintaining the same EPAP which can actually lower your average pressure. For example if you need 7 EPAP to prevent obstructions your average pressure will be 7 cm if you use no EPR but will increase to ~8.5 if you need/want 3 EPR.
Aerophagia is complicated because there is no single right answer as to what will work to improve it. Usually decreasing pressure is the key but it gets complicated with EPR having effects as well. Lowering both pressure and EPR can have negative effects on your treatment so as mentioned before treating both apnea and aerophagia is a game of compromise.
Since your pressure doesn't fluctuate that much I would recommend trying fixed pressures to help you more easily determine how changing settings affect your aerophagia and apnea. You can use fixed pressure by setting min and max pressures to be the same. I would start by trying 9 cm with 2 EPR for a few nights and then 10 cm with 3 EPR (both give the same EPAP). If you like the 10/3 better then try 9/3. If you like the 9/2 better you can consider trying 8/2 or 8/1. Try changing one setting at a time and seeing if you can figure out what gives the best results on a whole.