After trying and failing many times by using a normal humidity range to trigger the fan, I've been very happy with the Generic Hygrostat add-on available through HACS.

The problem with setting up "turn on the fan when humidity is above 65%" (for example) is that in the winter, when it's raining, etc... that might be the normal humidity inside the bathroom. Additionally, if the same humidity is used to trigger the fan "off", the fan will likely cycle too frequently. This may or may not be a bother to you.

The Generic Hygrostat (apparently there's a different Generic Hygrostat built in to HA, but it is not as good, so use the HACS one), takes an average of recent readings and sets it as the target. It triggers the fan when the humidity rises above the average by whatever percent you set. So, if it's 65% humidity on average, it won't trigger the fan until (for example) 70%.

One other thing I struggled with was cheap humidity sensors. Inexpensive options seemed to top out around 80%, and were not very accurate. I've had better luck with a Bosch BME680.

All in all, this automation was the most difficult that I've tried to nail down so far. It's working well now, but the problems which I've mentioned above took a long time for me to work out!