I only have an infrared thermometer, the one linked, which can obviously only be used for manually checking the temperatures. I had this already and didn’t buy it specifically for checking flow temperatures. However, I would imagine you could potentially buy a digital pipe clamp thermometer and connect it to an ESP32 to feed the values into Home Assistant.
It’s not something I have looked into, although I am interested in finding a similar solution to monitor the temperature of my hot water tank to see how much ‘charge’ is in it and automatically boost the hot water e.g. after someone has had a bath and mostly emptied it. I haven’t found a good solution for that yet, mainly as my hot water tank is one where the insulation is integrated into it below an exterior shell, so very very thermally efficient, but not easy/possible to get a thermometer onto it without compromising the shell. A pipe clamp on the exit flow might work, but wouldn’t be very accurate I don’t think, unless water was flowing through it. Therefore, I’ve actually temporarily abandoned that idea and am thinking of using a humidity sensor in the bathroom and ensuites instead i.e. when the humidy has increased 20% above the ambient humidity of the house, someone has either taken a shower or had a bath.