Temperature Sensor Selection for temp range of 160C to 240C with 1C Accuracy
A thermocouple is probably the best available sensor for that set of requirements but it requires careful design of the cold junctions and associated CJC sensor and thermal EMFs. No guard rings are required, thermocouples are very low impedance sources. 

If you can electrical connect the junction to the mass being controlled you will get faster response but you may have to float the front end etc.

1 degree C absolute accuracy is difficult to guarantee (remember that includes the wire limits of error, extension wire matching, any plug, sockets, cold junction sensor errors, as well as the amplifier and ADC and linearization errors) however, with careful design, stability of &lt;&lt; 1’C is not so hard.  

There are plenty of well designed commercial controllers that include isolation, and a lot of really crummy cheap ones on the market, you could consider using a decent one rather than going component-level for this common function.

You could also consider a thin-film RTD if you could couple that thermally and fit it. The design job for the circuitry then becomes almost intern-level.

As an aside, you should take care that the thermal coupling of the windings inside the cartridge heater is as good as possible to the sheath, otherwise you’ll degrade the control response considerably.