July-September is a bad time to release any Mac with the macOS upgrade cycle dates the way they are. When macOS n+1 is in high distribution beta from June-Sept it pretty should be a "all hands on deck" for doing development work on that version.
Kinda. Generally Apple tries to ride the train for whatever OS release will land just before the expected release date of the product and bundle in the drivers with that release. 10.14.6 was one such release they could have targeted, but with Catalina being "early Fall", there wasn't much point, as you say.
If we were going to see the Mac Pro on Mojave, we'd have started to see either a 10.14.7 beta with some "suspicious" new drivers, or have seen those drivers in earlier releases.
July-Sept is generally a bad window to release hardware products if you goal is to entice folks who just got bonuses, Christmas splurge purchases, and new fiscal year spending.
Getting iOS and iPadOS out the door is probably taking more time than planned too. There is another whole OS stack to validate and ship now versus previous years.
This doesn't really parse, since iPadOS is a branding change and statement of intent for the iPad, not a big engineering change. It's not even "another whole OS stack". The iPhone is still carrying the same SDK as the iPad, which includes the new UIScene stuff driving multi-window on the iPad.
iOS/iPadOS is practically the poster child of Responsive UI Design at this point. And Catalyst now adds macOS to the mix.
When iOS was younger, bigger iOS feature push would slide macOS . It wouldn't be surprising if that is kicking in again now
This is closer to what I think is going on. You've got some deeper changes in iOS in the form of Sidecar (for performance), multi-window support, opening up of the filesystem (and standing up drivers needed for thumb drives), etc. On the Mac, you've got Catalyst and SwiftUI landing at the same time which have to be working right out of the gate. This is probably one of their more aggressive cycles from an engineering perspective in a while in the sense that there's quite a bit of meaty V1 stuff landing all at the same time.