I am just finishing a build where I was facing the same question and I have two temp sensors in the end. My thoughts were that the fans on rads that I am regulating, are not actually cooling the CPU or GPU but the water therefore I need to monitor the temperature of it.
I also have a flow sensor since I cannot see the computer itself so I cannot say whether the pump is moving water but that is not really necessary because most pumps have an RPM sensor built in. But I kept it for increased redundancy.
PS: My build - https://builds.gg/builds/under-ceiling-build-18405
