I believe you're aware of the discrepancy between impl 'a + ... and dyn 'a + ... since you suggested the Captures<'_> workaround. Isn't the fact that you can't use bounds just a reflection of that? If you can't unify the lifetimes using covariance, you have to mention both lifetimes, just like you would have to with TAIT.
More on this in the detailed explanation section of this URLO comment for the uninitiated.