Svelto.ECS is a new platform agnostic ECS framework. The Vanilla example runs on .Net Core and .Net Standard, so I don’t see why you couldn’t use it with monogame. If you do, it would be great to know how it works!
I know this is a very old thread, but I thought I’d throw my opinion up for anyone who stumbles upon it like I did, since I think there’s a relatively new contender. After writing my own initial ECS, I was searching posts in the forum and doing lots of research to switch to a more battle-tested one. I highly recommend DefaultEcs.