@Nicschne You might listen to the two Flo podcasts with Steve Neal. He talks about a tempo / sweet spot progression with a heart rate cap of 82% of max.
He starts you off at 3x20min of SST but with the HR cap. If HR drifts up, power needs to come down. The goal is to get fitter to the point that HR doesn’t rise and power doesn’t need to come down. Once you can do 3 of those workouts a week separated by easy endurance, you move up to 4 per week, 2 blocked in a row, separated by easy endurance. Advanced athletes might even move up to 5 workouts per week or 4x20 or 3x30.
I thought the HR cap seemed like an interesting way to go rather than do the traditional way of increasing time of intervals.