Something wrong with your save data error caused my Xbox to become corrupted and is now bricked

You possibly picked the wrong storage device, either that or your data is now corrupted which will not be good. Just go in and look at your saved items on your flash drive and it will tell you if the saved data is corrupted or not

"I now have constantly error messages saying "something went wrong with your local saved games"

Have a look at this support, Cloud game saves FAQ

You must have a regular connection to Xbox Live to keep your data in sync. Playing offline may cause your cloud data to become out of sync. Your data updates each time you connect to Xbox Live.

I would check your network connection and use wired ethernet instead of WiFi.

If your network connection constantly disconnects and reconnects due to signal interference your saves will be out of sync between the cloud and local.

Until they join your game and cause your save to become corrupt.

It seems that something in the saved datas in the cloud is causing corruption. Probably a training, but I haven’t identified it yet.

There are many reasons for why this is happening, one reason that comes to mind is that there is corrupted save data: It's possible that the save data for your Minecraft game on your main profile has become corrupted, preventing the game from loading. Try deleting the saved data for Minecraft on your main profile and start the game again.