Hi all! I found the solution to my problem. Albeit being very specific to my case, I'm putting the information there in case it's useful to someone else.
So, my 'wake to black screen' issue on Catalina 10.15.4 was related to the modified version of acidanthera's BrcmPatchRAM I was using to make my DW1820A work.
These problematic drivers on 10.15.4 were available on another forum.
Since they are the only drivers that work correctly with DW1820A today, there is no more readily available / fully-working solution for this particular card on 10.15.4. AFAIK, this modified version of the drivers was loading a more recent FW to the DW1820A than the one still used by the official BrcmPatchRAM.
I switched to a new DW1830 card and used the latest official release of acidanthera's BrcmPatchRAM. And now everything is working mostly smoothly: sleep/wake, wifi and bt.
So, even if you don't have a DW1820A card, if you have this wake to black screen issue on 10.15.4, it may well be related to your wifi card + driver combo. Make sure to update to the latest version of acidanthera's BrcmPatchRAM.