What you're trying to do is, unfortunately, not that easily accomplished. Boundering grades are always a little subjective (depending on the climbers strengths / weaknesses, body mechanics, proportions, etc), so you may find that you struggle on a V8 whereas you fly up a V9.
I remember reading an interesting article a while back that the best boulderers in the world were reaching a point where they were setting very high grade problems that, completely unintentionally, may be close to impossible for other people to climb. Someone like Adam Ondra has a particular technique that he's trained to work for his body type, so someone who doesn't have that exact body type, may simply be unable to complete the moves to complete the route.
Having said all that, your edit about videoing your technique is a very good idea, but I would add in one thing.
I'd create a workout log style record of the routes and record two additional values; Rate of Perceived Exertion (RPE) and Rate or Perceived Technique (RPT). Both on a scale of 1 - 10.
RPE has been fairly well explained elsewhere, but is essentially how hard the route felt (so an overhang on crimps is going to use more exertion than a slab on jugs).
RPT is something I've taken from the Gold Medal Bodies (GMB) training programs. The idea being that you can rate your technique on a scale of 1 to 10, based on how it feels (and possibly on video review); so if you're ascending an overhang, and you're an upper body dominant, gorilla style climber, letting your feet hang free and swinging from jug to jug, then you're technique is going to be pretty low. If you're moving up the same overhang, but twisting your hips into the wall to increase your reach and heel hooking / knee baring to chalk up, then obviously your technique is a lot better.
The by-necessity variation in bouldering routes means that unfortunately, there's never one metric you can measure to gauge improvement. You know where your weaknesses lay, so maybe monitor those as you train them as they should show the fastest improvement.