The normal decay timer when you create the burrow can be disabled by the ini option in GameUserSettings under [ServerSettings] that already exists:
DisableBurrowDecayTimers=True
This mod disables the decay timer that still appears after using the gateway burrow and while at it, also disables the other decay timer described above so you don't have to add the ini option anymore.
This mod creates a new Burrowbuck/doe in order to link the changed burrow to it, so it will not work on already existing or tamed creatures.
I only now found out I could also try to add a buff to creatures instead of making child bps, so I will try that soon and hopefully make this mod work without making essentially new versions of exiting creatures.