Germany was a parliamentary democracy at the time, are you asking if id have voted for the SDP or KDP?
Precisely yes (or not voted at all if you felt no party really was representative of you properly)
Are you suggesting having two parties split the NSDAP opposition vote is what lead to their accent to power?
Yes, in addition to splitting the political energy in general
Or are you asking I'd be protesting Hindenburg to take more direct action against the NSDAP or more firmly address the crisis that lead to their growth?
I'm suggesting that the communists spending energy opposing the SDP and Hindenburg for fairly valid reasons, when there were much more pressing threats to the safety and security of the entire world including themselves to spend that energy on, made their concerns about the establishment left (however valid) laughable in restrospect.
But the thing that makes our situation so much different is that Biden isn't splitting the vote with another party, he's in command of the only opposition to Trump.
It sounds to me like you're saying that splitting the vote between Biden and nobody (by not voting) is a good thing to do, to push him to the left. I fail to see how that is a better idea than splitting the vote between the SDP and KDP, and I think the results can potentially be pretty similar.
It sounds like you really, really don't want to answer this question plainly. Would your logic also apply to refusing to support the SDP (or for that matter Hindenburg) against Hitler?