I double-checked and need to amend: at the time I in fact tried a number of tweaks to bluetooth power management but I had no success… so I finally made it like so:
sudo systemctl stop upower
sudo systemctl disable upower
sudo systemctl mask upower
(note: masking is important, otherwise upower gets enabled again by the OS)
Of course this “solution” would probably be less viable on a laptop, but I found it acceptable for my iMac desktop use.