Success.
Eventually found a 8-in-1 USB hub that can hold a M.2 SSD and has a built-in SD card reader (Orico OM28P-G2), to which I added the required SSD (1TB Kingston)
Connected to my phone, with an adequate explorer (Cx File Explorer) I can see both storage devices (HUB is the label on the SSD):
The performance is a rather pedestrian 26MB/s when copying from the SD card to the SSD using the phone (much better perfs when connected to the PC: around 250MB/s) but that should be enough for daily backups.
The setup is a noticeable battery drain on the phone but the hub has a USB-PD input that can power the hub and will also power the phone.