How to install older version?

Open the App Store, and click on Purchased.

Locate Final Cut Pro on the list of your purchases, and click Download. You will be warned that the current version is not compatible, and asked if you want to download the most recent version that works in your system.

Oh, wait... 2017 iMac?

Unless there is a specific reason you want to run High Sierra on that mac, you can upgrade all the way to Big Sur and use the latest versions of FCP and everything else.

Of course, before you do that, make sure to have good backups, and that all the applications you need work or can be updated for the new OS.

Again, do a backup first!

A little (big) thing to consider: if your mac only has an HD (or even a "Fusion Drive" with a small SSD), the performance with Big Sur (especially when booting) may be underwhelming.

Your mac is very capable but the 5400rm HD is slow.

In this case, it is worth considering an external SSD. It will make your mac fly.

This is what is called a "Fusion Drive". This is seen by the user as if it is a single drive. You don't specifically put things in either the HD part or the SSD part. The system will try to put things in a way that helps with performance.

You just install the OS and let it manage where things are.

If it proves to be slow, the alternative is to buy an external SSD, connected via USB3, and install the OS there.

This is in principle much faster than the internal. People who do this can use the internal drive for extra storage.

Make sure to have a backup first. Time Machine is dead easy to use.

Open the App Store and search for Big Sur.

Thanks for reporting back.

I love it when a plan comes together 😁