If Biden is pandering to far leftists, I have no reason to discount that his administration would not be a far-leftist administration. Keep in mind that Biden lacks the mental capacity to make actual decisions, so whoever is his VP will likely be the one to actually making decisions the president would normally make.
If Biden is pandering to the crazy leftists, he will likely turn off moderate voters in the 2020 election. The response to the coronavirus and the civil unrest has shown Democratic leaders' tyrannical tendencies, which I believe will hurt Democrats down ballot.
It's possible that he, or his VP rather, go completely deranged and start implementing unpopular policy but I think establishment democrats won't let that happen. Nancy Pelosi's done a decent job not letting the progressive newly elected members (AOC, Rashida Tlaib, ect.) from shifting the party even further left through any significant legislation that's made any sort of difference. I'd assume Biden's administration would be the same way because far left legislation WILL get out independents to vote Republican down the line in 2022.
Following Obama's election in 08, Republicans won in an absolute landslide in Congress during midterms, maybe because of Obamacare.