Mael
And the following apps are disabled on my GOS:
Disabling the calculator is fine, but disabling the rest can be problematic. Disabling Gallery is only fine if you installed a Gallery app providing all the standard intents. Dialer and Messaging can't be disabled in the Settings UI, but you've bypassed that by using ADB, and there may be consequences from it. The same thing goes for Contacts. It's on you to deal with any issues you encounter due to invasive configuration changes you're making via ADB. We can't provide support when someone is doing that.
1/ Why it's not possible to disable the default File Manager (com.android.documentsui) ?
It's a core OS component providing large parts of the UI and other functionality.
2/ Is it possible and safe to remove these apps with "adb uninstall ..." ? If yes, no problem with future GOS updates ?
It doesn't make sense to do this and it's not a supported configuration. If you do this and your device stops working in the future, we won't be able to help. ADB is for developers and is ideally only used on dedicated development devices. Use the Settings app's disable toggle instead of an ADB command. Settings app at least stops you from disabling certain important core OS components.