Spoke with Apple Support again.
They connected me with a Senior support specialist, incredibly nice and patient, that agreed that this is 100% a software issue.
We did try some troubleshooting.
This was a standard phone call and, of course, the Airpods didn't disconnect during the entire duration (they always behaved better on network calls). We did some tests, I walked around my place a few times during the call to try to trigger it again.
What emerged during the call - I wasn't aware of this - is
that there is a difference if you factory reset the iPhone from the device itself, or if you trigger a factory reset from a Mac's Finder with the iPhone connected via USB. I completely missed this detail previously.
In the first case the OS files are kept. So any misconfiguration or glitch that happened within /System stays even when you create a new user on the iPhone "as new".
Resetting from the Mac completely wipes the storage on the iPhone and installs everything from scratch from Apple's servers.
Immediately after the call with support, I did some other calls on Whatsapp and got the disconnections again, and decided to try and do the reset once again.
So, yesterday evening, again I took screenshots of all my apps, notes about the way I configured my iPhone, exported all the authentication keys and did a factory reset while connected to my Studio, it took a while. Then patiently re-configured it all and went to sleep
I really don't want to jinx it. Had about an hour of calls today, no drops.
I still have to try the other bluetooth devices (the motorbike thingie being the one with most issues).
Let's see what happens in the next few days.