Sorry to dive back into this a few weeks later, but I'd found his post really interesting:
If you register the chain as gear and set it to be automatically included in your ride. Then Garmin Connect will automatically keep track of mileage, no spreadsheet required. Assuming of course you use a Garmin GPS on your rides.
However, delving into it, like a lot of Garmins great ideas, this seems to be poorly executed. If I'm looking at this correctly, then if you only run one bike then it works seamlessly, however it's unable to differentiate between multiple bikes, so I can't track independent cassette usage for example on both my Gravel bike and Mountain bike.
Then a couple of days ago I noticed that the same option was available on Ride With GPS. Unlike Garmin, they have implemented it really well so I can have both my gravel bike and mountain bike assigned and then I can individually assign maintenance to these bikes. Because the Edge Explore 2 only has two default bike profiles (Road and Off-Road), I need to then edit the activities I've completed to accurately reflect which bike I was using, but it takes a few moments and then it's done. Then if I add in the date when I changed the chain for example, RWGPS will keep a tally of how many km since that point.
So no spreadsheets required and a lot of functionality with the RWGPS equivalent, so I think I'm going to be making use of that from now on. Thanks a lot for bringing it to my attention, even if the Garmin one is limiting for those that use more than one bike.
I should add though this is only available to Basic or Premium members only.