I just picked up a mac dev contract and it is pure swift while all other previous projects were in objective-c.
I think it’s possible to make a pure swift mac or ios app now. Beforehand I don’t think it was possible mainly because swift was not mature enough. I would say swift hit maturity at 4.2.
Even now, there are situations which can only be solved via an obj-c class bridged from swift. One situation I can think of is that you cannot catch an exception generated by the obj-c runtime in swift(as of 5.2). But a sneaky way of handling an exception would be to bridge-over to obj-c and catch it there using an obj-c class(this can happen during occasions when you are using an older API like key-value-observing, an api written purely in obj-c and is not handled perfectly by the swift runtime). However, this kind of thing is becoming more and more infrequent. Also, many very useful libraries(in the de-facto standard cocoa lib packaging framework, CocoaPods) are still in obj-c, which means you’ll have to bridge to the obj-c runtime.
Catalyst is probably better for more simple apps. Mac apps are targeted at pros and so you usually have “pro UI”, like multiple windows that is mated to a very high degree of concurrency and battery-draining I/O. So a software platform that is aimed at “serious” users will probably perceive Catalyst as a sort of compromise.
SwiftUI is not ready for production use. If I were a seasoned developer who never learned swift, I would focus in on a specific language feature like Combine. Combine is Apple’s RxSwift. I think it would help you learn Swift itself faster(albeit might be more frustrating at first) because you will hit the areas you need to learn right away.