There are a myriad of ways to do what you are trying. The simplest is to be sure the booster runs during a time the filter pump is at a higher speed. I've been doing that with my pool for over 10 years with no issues. As I have an electronic timer (Intermatic P1353ME) for the booster, it will save time settings for up to 8 hours is the power goes out, similar to a VSP.
I actually have one of those current-sensing devices in a drawer in my office. The problem is that it only senses current, so relying on it solely is not the best. I took it off a pool where it was allowing the booster to come on, as it sensed the filter pump was on, but there was no water flow. The customer seldom looked at their pool, but both the skimmer and pump baskets were clogged. They thought that the device would allow them to not have to worry about their booster anymore. They had me remove it and learned an expensive lesson. They are not particularly expensive.