(also a Brit)

Biden was no longer a candidate even by the time the last election happened.

Look to Mamdani. Note that the real election there was in the primary. If you squint a bit, the US electoral system looks like the French one. There's two rounds of voting, and in the first one you get to pick who is the crook that will be put up against the fascist in the final round.

It's going to be boring and time consuming, but people have to use the levers they do have available to do internal Democrat party politics if they want to improve the situation.