We pretty much use desktop for email, web browsing, watching YouTube, download some files, occasional video calling with doctors.
What will we be able not to do with an iPad Pro vs having a pc or mac?
For those particular purposes alone, I'd say you'd actually be totally fine with an iPad (other than that the screen will be smaller than what you've accustomed to, esp. on the lower-cost options since the larger iPad is only available as the expensive Pro variant).
You'll start to run into issues if you spend more time creating things, e.g. writing more complex documents, creating fine-tuned presentations etc – and the problem isn't really that you couldn't do those things at all (there are e.g. perfectly fine office apps from MS and Apple), it's more about that they might not have all the features you're accustomed on a proper computer and many things end up being surprisingly time consuming to do, which would've been a breeze on a regular computer. Take this from a guy who some years back tried to solely use an iPad Pro as the on-the-go computer (while having an iMac at desktop for more elaborate tasks) – gave up after a year or so and to a tiny 12" Macbook. Apps have developed further since that for sure, but the same root cause is still there…
Of course there are also plenty of things that an iPad CAN do that you wouldn't be able to accomplish on a laptop/desktop, e.g. drawing or taking notes with the Apple Pencil – and many things that are just way more FUN on an iPad (e.g. I love photo editing on iPad in Lightroom compared to desktop even when the screen is smaller, the direct manipulation just feels great).