Yeah Zelle doesn't really work well for that because I think you can only ever have one Zelle-enabled account because it's based on phone number or email.

Some banks offer external bank transfers through their online portal, and sometimes with a small transaction fee. I've been told by my bank with that transaction fee that you can avoid it by using the "bill pay" feature and just inputting your other bank account's routing and account information. I think that all works over ACH anyways.