Seems i'm stuck on this open issue: [Github] Piwigo issue #2078

"The synchronization mecanism does not generate pwg_representative files (ie "preview"). That is perfectly what it expected from the sync : going as fast as possible. Generating representative images is slow. So it's not included in the sync. There nothing new or specific to HEIC files. It has always been this way with other "non visible on web browser" files, like videos or TIF files.
What is indeed problematic is that there is no way in Piwigo to re-generate the representative. I agree we should have that action in the Batch Manager.

For now, you can add your own pwg_representative for each HEIC file, before any sync."

How would I go about that last step?

For now, you can add your own pwg_representative for each HEIC file, before any sync."
How would I go about that last step?

You might try the Photo Update plugin, which enables to add a representative from your computer when editing a photo. I am not sure for HEIC files, I do not use any. But I do it for all my pdf files.