I have two brewing dial thermometers that I have equally calibrated with ice water. When I reach mash temp they show different numbers ± 4°F .
If you can calibrate your thermometer then you should calibrate them at the temperature which is most important to you, in the range that we care most, which is from 55°C which is a possible mashing step when using moderately modified malts to 78°C which is the usual mash-out temp.
That means borrowing a trusted and precise thermometer only once.
Your calibration temperature should either be 63 °C - 145 °F which is a typical monostep mashing tempearture or a bit higher, 66,5°C, halfway in the abovementioned range. That should guarantee a high accuracy even with a cheap thermometer.
That said, I use this cheap thermometer:
https://www.amazon.it/gp/product/B085ZTWW56/ref=ppx_yo_dt_b_search_asin_title?ie=UTF8&psc=1
€16, gives a correct answer within a few seconds, decently splashproof, auto on auto off, very useful for all sorts of measurements such as water for hydrating dry yeast which I warm up with the microwave oven, or to check mash temperature at different points of the kettle (which gives quite discouraging results... don't do that, ignorance can be a bliss
).
Generally speaking I am surprised by how coherent the various thermometers I have are, and how variable is the mash temperature in various spots of the mash. Mash uniformity is IMHO a much greater problem than thermometer accuracy.