In the US, where in most places the general election is preceded by a "primary" election for the parties to select candidates for the general election and in which you can only vote for candidates from your party, I'd suggest one other thing when it comes to voting.
If you live in an area where one party pretty much always wins in the general election, register for that party so that you can vote in their primary. That way your vote does actually have some influence on the outcome.
It won't change which party wins the general election, but it can help move the winning party toward the center. Primaries are usually lower turnout than the general election, which can make it easier for an extremist candidate to win them because the people who vote for the extremists tend to be more enthusiastic and thus more likely to actually vote in the primary.
If you are in an area where both parties have a good chance of winning the general election, but one party seems more susceptible to extreme candidates winning their primary, consider registering for that party to try to move them more toward the center. It means giving up influencing the other party, but if that party usually picks OK candidates your vote is probably better used trying to help get rid of extremists.
So no the idea of a primary challenge is a lost cause and the younger generation internalized it this past election.
But after that, an EV, solar panels and heat pump heating are 3 steps that will eliminate hundreds of tons of CO2 vs alternatives, and will increase your standard of living since they're cheaper over the long term.
Going vegetarian and eliminating flying also would have a big impact, but that involves real sacrifice, unlike the above options.