iMac running slow

Thank you. Good info. Please connect the backup drive, turn off the scheduler app, and let a few backups accumlate under normal TM management. Then do another EtreCheck report. Some external drives, particularly some "name brand" drives, some with odd formatting schemes, can run very slowly.

If you have a quality external drive drive enclosure rated USB3 that contains a 3.5-inch 7200rpm drive rated at SATA 6G, incremental Time Machine backups are complete unobtrusive. I also have a 2017 iMac and no backups are slow. I use OWC 's Mercury Elite Pro enclosure fitted with a fast WD "Black Series" drive. Happy as can be with their performance.

Previously I had a WD "My Book Mac Edition" that was a horror story at first. It slowed booting, caused stalls at odd times and made the computer run hotter. I reformatted the drive to a conventional Mac format. That helped marginally but did not entirely succeed in making the drive "get out of my workflow." That nonsense disappeared when I moved to the OWC enclosures.

Another report with the backup included may help us sort this.

Fortunately you are not plagued with the usual suspect, a slow and under-spec mechanical hard drive. Your Fusion drive is preforming quite well and within nominal values for similar Fusion drives in other iMacs.

Can you detail for us what tasks you are doing when the beachball happens?

Backblaze and OneDrive may be competing with Time Machine. You also have a process called Time Machine scheduler. Anything the slows backup s will slow the system while in action.