ACA was the single most impactful legislation passed since god knows how long, so yes, i would agree that it is the major things democrats have accomplished. another major accomplishment of democrats this century is getting out of the iraq and afghanistan wars. keeping the US at least reasonably in the picture when it comes to the green energy revolution (though we are still behind where we should be). how about dodd frank and CFPB? how about guiding the US into recovery thru the great recession and post covid? the latter of which has so far threaded the needed as far as necessary stimulus to keep things growing and has finally curbed inflation without tanking the economy?
while not specifically legislated, biden's NLRB has been one of the biggest boons to organized labor since probably FDR. even symbolically, he is the first president ever to walk in a picket line (UAW strike). while its been struck down for now, he also tried to outright ban non-competes. sure he's not trying to abolish wage labor, but this part of his presidency is pretty unequivocally left wing
in what universe is medicare for all a centrist position? yes you can find the odd poll where 70% of people or something say they want it but when those polls are phrased in ways that say things like "private health care plans would be eliminated" the support vanishes. if it was really that popular, we'd see way more M4A types get elected. bernie would have outpolled any democrat in primaries if it was that popular among democrats, let alone the general public. and yeah, while the DNC didnt do bernie any favors, he also wasnt getting enough votes despite M4A being is one calling card that everybody knew him for.