Sorry @tyxo, I do not have experience of this fairly “modern” device, to say how “simple” would be the sideload. I think your Upgrade method should include matching upgrade of e-recovery; that’s what I would aim for.
Perhaps unzip the T ROM to see / confirm if it includes e-recovery; I think it should. Check that your device is set to update e-recovery with each update. Settings > Search Update recovery in Developer options.
In general “T” builds have been engineered with vendor parts to do the S → T Upgrades without need to Format data or “return to Stock ROM”.
Always have a backup of essentials when contemplating a significant intervention.