Good question. I tested your theory by calling my husband who usually answers his iPhone calls via his hearing aids. When making or receiving a call, the Audio button allows him to pick one output. If his hearing aids are connected to the phone via Bluetooth, then he cannot select Speaker without turning off his hearing aids. This is true for me even without hearing aids. When I chose one output, it turns other options off. So even without hearing aids, I cannot listen to a call (or music or a facebook reel) on both Speaker and hone at the same time.
FYI ... we both have iPhone 17 Pros, updated to 26.0.1 or 27.0.1.
So to answer your specific question, no need to get a new phone. You will not be able to do what you want on your current iPhone or a newer.