I've found Crystal Disk Info to be very informative. When I last replaced a full set of NAS drives proactively, I checked each drive and posted an image of the results with the Ebay listing for the ones I sold.
While none of the drives individually or the array as a whole reported any problems from Synology tests or in logs, Cystal Disk Info did identify errors on a couple of drives which.
Crystal will require taking each drive and connecting it to a computer to run the tests, but the results come back very quickly (seconds) which suggested that the information it reported is logged but not reported by other utilities I'd tried, and that the testing itself is not stressful for the drive.
If your drives are ~5+ years old and you want peace of mind, it's worth a try.
https://crystalmark.info/software/CrystalDiskInfo/index-e.html