How can I stop a Level Sequence from resetting at the end ?

As of UE4.19 you can set the default to Keep State.
What's New | Unreal Engine 5.1 Documentation (search “When Finished”)

Should be in DefaultEngine.ini (if not, try other ini files)
[/Script/LevelSequence.LevelSequence]
DefaultCompletionMode=RestoreState → KeepState

Haven’t installed 4.19 yet so I haven’t tested.