What could be the cause of this?
Is there any way I can keep my configuration for 25.12, or will I have to reconfigure everything from scratch, including WireGuard and all my apps?
Do not restore config tar gz file.
/etc/config/network (vlans, vpns, routes, ip addresses) and /etc/config/system (ntp, swap, timezone) are not valid. But you can untar (tar xvpzf config... ) config tar gz in /tmp on the router and copy individual files back (except 2 mentioned you have to manually re-create)
Upgrading from 23.05 to 25.12 is not officially supported, there may be a reason why?
To recover wireguard and and "apps", just flash a custom image already including these stuffs.
This is the expected behavior, it is written why : your device works with DSA while under 25.12.
So you need to reconfigure the device from scratch. That being said, you can manually import some files or copy/paste data afterwards, so you can quickly find back a working device. Follow @anon63541380 's answer.