How to prevent frequent disk ejection errors on iMac's external backup drive?

Start by updating the iMac to 15.7.3

Then change the cable that connects the drive to the iMac. Use a quality cable rated for USB-3 and/or Thunderbolt 3. Use no adapters if possible.

There are any some apps or utilities that might cause problems. If there are anti-virus, cleaning security or VPN apps that have been installed on the Mac, it would be wise to uninstall these for the time being as you troubleshoot the problem.

Is the cable long enough so that the external drive can sit solidly on the table, but not long enough that the cable will be bumped or pulled or dislodged (even partially)? The USB port cable connection may not be able to withstand having the computer moved, or the cable moved/bumped.

check macOS power management and how Time Machine interacts with that Seagate drive. On silicon iMacs, bus-powered HDDs are especially sensitive when macOS spins them down during sleep or idle, then tries to wake them for Spotlight or Time Machine, which can trigger false “improper ejection” alerts. Go to System Settings > Energy Saver and disable “Put hard disks to sleep when possible,” then exclude both external volumes from Spotlight indexing and temporarily turn Time Machine off to see if the errors stop. If they do, re-enable Time Machine after repartitioning so only one APFS volume is used for backups, as multiple volumes on a single USB HDD often confuse sleep/wake handling even when Disk Utility reports no errors

What type of drive is it: HDD or SSD?

If an HDD does it have its own power supply?

Which port is it plugged into?

As well as the other advice I would check the ports on the drive adn the Mac for lint and dust.

I had this problem when I plugged my external drives into unpowered USB hubs. Are you plugging directly into the iMac or via a hub? Since using powered hubs I no longer have the problem.