There is no way for us to know what iPadOS will have, or not, in it final, introduced form in the Fall of 2019.
There is no way to know which iPad models will be able to successfully access/connect to more conventional external drives other than flash memory drives.
Disussion and speculation of new or upcoming products and software is prohibited in these open and public Apple support communities, per the Terms of Use ( ToU ) we all agree to when we sign up here into these official Apple support communities.
iPads, even iPad Pros, that will be able to run the new iPadOS, will STILL NOT replace a much more powerful and feature rich laptop/MacBook!
AND, you cannot run full computer OS applications on any iPad/iOS/iPadOS.
In most cases, ONLY scaled down versions of well known apps!
iPads are NOT full replacements for any laptop/MacBook, yet.
Get an iPad Pro, if you wish and can afford one, but keep the MacBook!
Adobe hasn't even, yet, released their first version of full Photoshop for iOS and they have been talking about and working on this for years, now!
Like many of us, You'll simply have to wait a couple more months for the official release of iOS 13/iPadOS.