I set up automatic backups of WhatsApp to my self-hosted Nextcloud once. Since you need 'tested backups', I tried to decrypt these WhatsApp backups independent of my phone, but this was not possible. You need the original device. There are some hacks online, but they are always out of date.
I am tending now to running Mautrix Whatsapp bridge and backing up my data through this.
I am tending now to running Mautrix Whatsapp bridge and backing up my data through this.