I believe I saw something called “Mass” on UE5’s commits on git, it seems like an ECS system but can’t confirm. It looked very much like ECS to me.
there is also third party ECS EnTT/flecs you can integrate but I would suggest just optimize the code you have and you will be fine, even with ECS you won’t be able to have 1000’s of skeletal meshes as there is no Instanced skeletal mesh component out of box.
plus I think Unity’s ECS is just awful at this point but I am going to give them the point of trying and taking the step in right direction.