Does not exist:
Use apt to pin select packages as higher priority:
AptConfiguration - Debian Wiki
If you want to use the Librem 5 again, you must reflash. Before doing that though, you can attempt to backup your Librem 5 using Jumpdrive:
Whether you eventually reflash your Librem 5 yourself or Purism support does it instead is your choice. Either way, an external computer will be used to restore the Librem 5 to its original state.