Here's a afthumbs.py Python3 script which can be used to extract PNG thumbnail images out of Affinity files.
The afthumbs.py script can be called either by passing over a Affinity file as argument directly ...
... or in an interacting mode just like this ...
If the script does find a PNG thumb image it will extract it and write it into a pngs/ directory!
Here's the afthumbs.py Python3 script for download:
afthumbs.py
Have fun!