Sounds like lots of theory, and very little practice. Thermowells are commonly used in the industrial arena but, if they are as bad as you imply, they wouldn't be. And, if you are really serious about all this thermal mass stuff, you probably shouldn't be using a DS18B20.
"Sounds like" would be correct. The thermowells and instruments I have used in "the industrial arena" are substantially smaller in diameter that what it would take to (a) make my own thermowell, that could (b) accept a probe 1/4" in diameter. The thermowells I have worked with have all been smaller than the 1/4" diameter making the sensing device even smaller. Since we are talking about circular items, the volume of material is increasing r-squared.
The 7mm waterproof sensor shown above are hardly suitable wet in high pressure situations as there are no shoulders on it, thereby leaving you entirely reliant on the compression ring. This is probably not something you would want to walk too far away from. If you don't have any pressure, they are fine and, in that situation, I just put them in a standard retort bung.
I have been pondering this idea, a little. How much pressure would you consider high? 10psig? 20, 30, 50psig? (beyond 30psig, other things will break anyway)
I recall standard thermowells suitable for DS18B20s do not seem to be such a good proposition, and I found it simpler to make up my own.
What do recall as being the issue? Can't find on that fits the device? Sealing?