Caveats - I don’t have Phonak aids and I have to use an intermediary device with my Oticon OPN S 1 aids with my Macs so your results may very well differ from mine.
For reliable bluetooth from an iMac Pro running macOS several different versions of macOS, I have had to use a USB Bluetooth dongle instead of the built-in bluetooth (specified as 5.0) in the Mac. With an older MacBook Pro (2016) with Mojave, I can use the built-in Bluetooth (4.2) without a dongle.
You might try a Bluetooth dongle - Oticon included a Sennheiser BTD 800 USB ML with the ConnectClip intermediary device and suggested it be used with any desktop PC and notebook if they had bluetooth issues. I’ve read where a forum member was able to connect Costco aids (based on Phonak) with a less expensive dongle. I have read that the implementation of Bluetooth hardware/software is sometimes incomplete/buggy such that some devices won’t reliably connect with those implementations. Sennheiser makes headsets for use with PCs in call centers so I expect they make the dongle to ensure their headsets work correctly with PCs without Bluetooth or with flawed Bluetooth implementations.