I cancelled my Zwift subscription recently. It's not that Zwift is bad, I just don't use it often enough to justify the monthly price. I've been looking for a free alternative that I can use whenever I feel like riding inside (very rare these days).
But then I found BullWatt in a Reddit thread. It's exactly what I want, even though I didn't realize it at first. It runs locally in your browser and uses the Bluetooth API to connect to your trainer. Unfortunately, it appears that only Google Chrome has any decent support for this API so I had to install that first, which pains me to see it next to Firefox.
The best part is that because it runs locally in the browser, there are no accounts or sign ups. You can just start using it immediately. Absolute cinema.
I just finished a 25 minute session and it did exactly what it advertised. It let me pick a workout and set my FTP, and then it used the ERG mode of my trainer to automatically set the resistance.
The source is on GitHub. I'm going to poke around there later to see how it works in more detail.