The leap between 6 and 9 is significant. The biggest change from 8 to 9 was the new mask system. Yes, AI is the most-mentioned feature, but the whole masking engine got an overhaul which makes the other mask types more powerful.
Regarding PL10, experience says it will be released in the September-October timeframe, so you’re about 5 months away from that. Of course, no-one knows when it will be released except DxO and they may change things up this time.
PL10 will certainly have new features (otherwise it’d just be 9.x) but you won’t know what they are until it goes on sale. You might care about masks and nothing about those will change. You might care about the DAM and nothing about that will change. You just know there will be some changes.
What I would point out is this: you’re outside of the upgrade path, so there is no financial penalty to waiting. If you were currently on PL7, then you could get PL9 for the upgrade price, but not PL10. As it is, you’ll be paying full price whether you buy PL9 or PL10.