Hey there Shelbatron1000,
Thanks for reaching out to the Apple Support Communities. We understand your VO2 max is decreasing despite increasing the intensity of your workouts, and we'd be happy to provide you with the best details.
To get started, how long has it been since you set up Cardio Fitness Levels? We ask because depending on conditions and medications it's possible that your VO2 max may have been overestimated, and it can take several several workouts and measurements with your Apple Watch before a more accurate VO2 max is determined. Fore more details, check out this link here: Set up Cardio Fitness Levels - Apple Support
We'd also recommend checking out this additional resource on how to calibrate your Apple Watch for improved workout and activity measurements: Calibrating your Apple Watch for improved Workout and Activity accuracy - Apple Support
If you continue to have issues with the VO2 max decreasing unexpectedly, we'd recommend reaching out to Apple Support directly from this link here: Get Support - Apple
We hope that's able to help.
Take care!