iMac fan running continuously

Nope. That Mac's CPU is loafing. That implies something else is wrong, and if the SMC Reset did not fix it then contact Apple: Official Apple Support. It could be something as simple as dust obstructing the air inlets, or the fan blades, or both.

Also, have a look at the pictures posted by Allan Jones in this Discussion: iMac Maintenance - Apple Community. That sort of thing just strikes me as common sense, but apparently it's not as common as I would have thought.

hoffmaj wrote:

Re: iMac 27 inch 3.1 GHz Intel processor, 2019

The fan runs continuously.

That iMac's fans run will operate whenever it is not sleeping or powered down. It should not be objectionably loud though. Read About fans and fan noise in your Apple product - Apple Support.

If you want to investigate further post a screenshot of Activity Monitor, with either the Energy tab or the CPU tab selected. Sort the respective columns in descending order. Post a screenshot. Energy-intensive or particularly demanding tasks have a direct correlation to the amount of heat produced that has to be removed by the exhaust fan.

I have TG Pro installed to try and better manage the temperature.

Uninstall it forthwith. Using clever things like it are guaranteed to ruin your Mac.