Backup is a problematic topic on Android.
Private app data cannot be accessed by design (sandboxing). Third-party "backup" apps cannot access this (without root privileges). Any app is responsible for providing an export function.
Google cloud backup cannot be used when migrating to GrapheneOS (no way to restore).
Files like photos should be easily accessible with MTP – unless it is acting up. I personally have had bad experiences with MTP. Whenever trying to copy many and/or large files I ran into trouble: Sudden errors, disconnects, device cannot be reached…
Try multiple USB cables and multiple computers.
On this forum the advice is usually to not use developer options, especially USB debugging. But pulling files with ADB is also an option.
You might have better luck with backing up files via WiFi (maybe Syncthing?).
In third-party operating systems such as GrapheneOS there is Seedvault as internal backup solution. The results are… let’s be polite… less than ideal.
Good luck!