Minimum solder mask ridge (sliver) for DFN package?
It is a good idea to have a solder mask ridge between pads, as you say, to reduce the chance of short-circuits. It is not very important that the thermal pad on the PCB is exactly as large as the thermal pad on the component. With a smaller PCB pad you will most likely still get a good electrical connection and good enough heat transfer. (The exception would be when you are trying to use the component near to its maximum temperature and power.)

So, if you are not using the component near its thermal limits, reduce the size of the PCB thermal pad to allow a solder mask ridge around it.

Although not a direct answer to your question, another alternative is to use a smaller solder mask expansion. Perhaps 0.05mm would be enough depending on your manufacturing process.