It's likely there will be:
A spec improvement to the current iPad Pros in October.
A new 10.2" budget iPad replacement in October.
New iPads in 2020 (either Air 3 / Mini 5 successors).
So depending which models you're interested in, make your decisions off of this. It's only a guesstimate... but until these events happen it's all you've got to decide on.
Personally, I say if you want one now, get one now. They're all good choices, will be supported for a long time and will perform beautifully for a very long time.