"over the last 20 years, solar activity has been slowly declining, which should have led to a drop in global temperatures if the theory was correct."

Solar activity has NOT been declining over the past 30 years, just the last 2 decades. A reasonable person might conjecture that there is a lag in any supposed 'cooling' effect, say one possibly resulting in lack of warming over the last 10.

My personal opinion is that the sun does have a role driving PRESENT DAY climate but it's minor. Anthropic CO2 etc. also has a role, probably larger, but still minor. Reducing fossile fuel dependency is simply a good idea for many reasons. Doing so in a manner free of UN politics and envirohysteria is an even better idea. Leting a Divinity School dropout drive international science and energy policy is a very bad idea.