I think we’ll see 13.5.2 maybe next week along with another 13.6 beta. June 22 will see beta 1 of iOS 14, and at that point we’ll start seeing less and less effort put into iOS 13.x
There is already close to non effort put into iOS 13! Minor bug fixes an optimisations, not more.
The development of iOS 14 does not START with beta 1, it goes into it's last stage!
The majority of the iOS software engingeers are working on iOS 14 at least since three or four months if not more. Don't forget that we get a BETA of iOS 14 on June 22nd. There's a lot to do before even raching BETA status (=the final state before the software is finished). There have been a couple of pre-alpha- and alpha-versions internally for months, that's for sure.
I personally do not expect iOS 13.6 to be released before June 22nd. More like end of June. They can have iOS14 and 13.6 in beta at the same time easily as there are different profiles needed on the device.