Can I upgrade the hardware or do I have to replace my iMac to continue to collaborate?

The current versions of Numbers and Pages require macOS 14.0 (Sonoma) or later. Apple set this requirement back when Ventura was still one of the "most recent three."

Apple has transitioned the entire Mac line to Apple Silicon processors. Only a handful of Intel-based Macs are able to run macOS 26 (Tahoe), and Apple has announced that Tahoe is the last major version of macOS which will run on any Intel-based Mac. If you get a new Mac, you almost certainly want it to have an Apple Silicon chip.

There are no 27" iMacs any more, so you might want to consider pairing a M4 or M4 Pro Mac mini with one or more standalone monitors.

Your 2017 27-inch iMac 5k can run no macOS newer than macOS 13 Ventura.

Upgrading that iMac to run a newer OS is not an option.

If you must use newer software that is not compatible with that macOS 13, then you'll have to acquire a newer Mac.

If money is a factor - when isn't it? - you might consider buying from Refurbished Mac Deals - Apple.

Apple's refurbs carry the same warranty as their new Macs and the initial outlay is somewhat less.

If you have Time Machine setup you could restore an older version of Numbers, Pages and Keynote. However, at best this is a bandaid approach and a messy one at that. The suggestion to consider upgrading to newer hardware is the best advice so-far. I would strongly recommend getting a new MM with adequate storage (the same or more as your 2017 iMac) and an external display.

You can recycle the iMac for free using one of Apple's free recycling programs. You can also migrate your data from your Time Machine backup to the new MM extremely easily if you follow the steps in Setup Assistant and Migration Assistant and you will have future proofed yourself for years to come. With technology it is wise to upgrade every 6-8 years or so as it moves forward quickly and using old technology starts becoming difficult as you are finding out.

Best of luck.

If you have been using Time Machine or another app to backup your Mac, then you may be able to recover an older version of any of those apps. Otherwise, you'll be limited to the versions currently available for download from the App Store.

I suggest that rather than trying to move backward, you simply make the effort to move forward and learn what it takes to use the newest collaboration features of the software to their fullest.

Collaborate on documents in Pages, Numbers, and Keynote - Apple Support

Intro to collaboration in Pages - Apple Support

If you have issues with the way in which the collaboration features are implemented in the apps, then by all means voice you dissatisfaction directly to Apple by leaving feedback using this link:

Product Feedback - Apple