Depending on the topology of the data, different metrics are used and you can make different performance tradeoffs. And differing business use cases make it hard to find a one-size-fits-all. As I'm sure you're aware, there are dozens of fast ANN approaches, and the code implementing the ANN often is the tightest loop, making hard to be pluggable and performant.
TileDB is also quite interesting.
Best of luck though, I agree with you there's a "missing product" to be invented, I'm not sure this is exactly it, but I don't think I know any better.