Hi Lanky,
How unfortunate your Synapse does not restore!
I have previously migrated from 32bit to 64bit, actually from ArmHF (on Orange Pi Zero) to Intel x64 (on Proxmox). I started with plain Debian, and added Yunohost and after that the backups. The backups restored with no problem. Even though that was more than a year ago, I am quite sure your problem does not have its cause in the migration.
It is exactly the same place as where @Totu has his error, and where I had it. @TheTrveAnthony had a backup of a much older Synapse, in his case the setuptools-switch seemed to have worked in this location (but he still had a metadata-error later on).
Is your old host still running/available? You might try upgrading Synapse to the latest version (your log states YNH_APP_MANIFEST_VERSION: 1.91.2~ynh1, I have 1.92.3~ynh1 installed). Probably won’t fix it, but worth a try.
I am not knowledgeable enough about Python/PIP to have an idea why this problem keeps popping up 