You can use aluminum oxide isolators.
AAVID THERMALLOY sells them under 4180G for example. But thickness and shape differ.
It isolates and transfers a lot better than mica or sil, at the price of fragility and cost of both bom and assembly.
https://www.shopaavid.com/Product/4180G
15W/mK, really, for what? You can get silpads of similar thermal conductivity and completely bypass the issue of fragility and assembly steps of applying thermal paste.
If a modern high-tech 10..15W/mK silpad isn't going to do the job, then I'd be seriously looking at spreading the heat so that it can be solved with said silpad. If you can accept down to about 5W/mK, you have a massive, proven market to choose from. Electrically isolating, safety-approved, robust thermal coupling just seems to be limited at around 10..15W/mK, if you need higher, it'll be very expensive and difficult, or risky.
Beware of brittle and/or very thin materials, which work fine in lab conditions and may have all required approvals on paper, but break down in production due to minuscule surface imperfections that either shatter the material (ceramics), or puncture through it (silpads or tapes too thin).
Increasing the surface area is often the easiest way.