I think the lack of keyboard shortcuts would be much more of a drawback on a tablet as PM's one of the main boosts in speed and efficiency is keyboard shortcuts.
Of course we aren't all the same, but I don't think typing texsts must be a real issue with an iPad. I have an iPad Mini and I type almost as fast on it as I do on my MacBook (what is not a slow one as I type using 10 fingers and can do it blind as well). I have had similar feelings against typing on the iPad before thus I avoided using it for typing longer texts (long mails, blog posts, articles, etc), but I got more and more used to it and I just observed I started doing most of my text production on the iPad.
Auto corrections can be a pain in the ass as well as great help if it gets trained well or turned off.
Also if typing texts on the iPad is still such a trouble for someone, I think quite some of the captions can be prepared well in advance not needing you to modify it or requiring only a small tweaking here and there. So you can write them on the laptop and copy+paste them on the tablet.
Actually I usually do the exact opposite. I write some or all of my captions on my iPad during boring jobs while waiting for the event to start (or end) and I send it to the laptop to use in PM.
I also prefer using TextExpander shortcuts on both the iOS devices and the MacBook which is a similar assistance to the Code Replacement feature of PM just it works across all the apps I use and it helps a lot with typing the recurring names, phrases, expressions or even parts of sentences, just like a lot of the photo captions. Using such a feature not only helps me to type faster as I have to type only a few characters instead of a full word or sentence, but it also grants the correct spelling if you managed to get it right in the first occasion when you entered your snippets.
There are some customisable keyboards for the iPad too where you can setup your buttons with custom text insertions (like hashtags for social media use) I'm sure one can find and set up quite a lot of the phrase elements to be used in captions, even the most often used names, places, etc.
While it would be cool to have PM on the iPad Pro, I honestly don't think I'd use it that often.
The main reason being how much slower it is to type (even with the smart keyboard case). I need full captions on anything I send back to the paper and I still think it would be faster to simply pull out my laptop with the regular keyboard then try to use the iPad Pro.
Obviously not everyone needs to type out full captions, just offering my perspective.