Your experience is on par with the consensus of other high mileage commuters.While changing gears is a contributor to chain wear I think for high speed commuting it is actually the constant high power that causes the wear. At higher speeds rider + motor output can easily exceed 400W for long periods and you wear the chain at a much faster rate. I am busy with moving these days but I am keeping a somewhat accurate chain wear journal (measuring with a digital caliper to see the chain wear progress) and will share it once I have
Recently, I was watching a video where Gates Drive engineer was explaining a scenario in Netherlands and how Gates drive was able to alleviate his problem.
This guy was commuting 60km everyday in Netherlands (flat as pancake), so he was spending 90% of the time on the last two cassette clusters (smaller ones) and he was tired of replacing them every 5 weeks.
Then they switched him over to gates drive and Rohloff and his maintenance intervals were much longer and less work.
On flat-ish terrain, chain spends disproportionate amount of time on the smaller cogs and wears them out quicker and in process wears itself out as well.