As long as the solder joints are good, then heatshink will do wonders for you. That bottom joint looks a bit dodgy, can't tell much about the other 2. People have been using the heatshrink method for years with thermistors.. Once the outer sheath is on you aren't likely to end up with failing joints or broken leads due to flexing, unless you deliberately try to flex it. If you're worried, then you could extend the sheath over the bottom third, or even the whole sensor - but you'll lose some time response dues to the insulating property of the heatshrink. Doesn't sound like your application has a need for 1 second response times though.