Mac won't boot of an external drive, starts internet recovery instead

Presumably your data on the backup is fine, it just doesn't have the correct boot sectors or they are corrupted. Why not go thru with the internet recovery, get your Macbook to a base state, then you can use Migration Assistant to move files from your backup to the now refreshed Macbook.

MacBook Pro 15,2 is the 2019 13-inch model. It has T2 chip in it. By default, it forbids booting from an external drive. Search for 'T2 chip secure boot' for more info.

It requires a valid macOS and Recovery to disable secure boot.

Currently you have to use internet recovery to install the OS, or head to an Apple shop.