Have you ever thought about trying a Polarized training approach? Here are some good resources for you to peruse if interested.
And some great Podcasts:
https://www.fastlabs.com/fasttalk51/
https://www.fastlabs.com/fasttalk54/
Some good articles on Polarzied over SweetSpot:
The TR Thread on the above article:
https://www.trainerroad.com/forum/t/sweet-spot-or-sour-spot/29948/140
And some TR threads on the Polarized topic which have some great insight from fellow TR users: