I will keep going with the intervals just to give me variety - it will take a while to really see anything I think, we shall see.
My VO2 Max used to be 29.8 (the bottom end of the 'good' band) and then when Apple Watch did the latest WatchOS Update it dived to 22-23 (which is in the middle of the 'poor' band). Lots of other people have had large drops too. I keep hoping it will improve but no. However, I think I must be reasonably healthy as I walk every day, cycle long distances, jog for 30 minutes. So I guess the original reading, which was similar to what my Garmin Vivoactive 3 offered, was more correct for me.
I am enjoying looking at the splits of my intervals in Strava to see how things are going - I seem fairly consistent in my running and jogging speed, they are both just pretty slow!