I think a key issue here is that you're combining unlike things and trying to make coherent sense of that, rather than analyzing what is driving people to feel this way.

The first part you mentioned, is a key disagreement you have with people opposing reformism. A significant part of leftist history is the conflict between reform and revolution, whether reform is even possible at a large scale or if revolution will ever be more likely to succeed, and so forth. The people opposing reform are not saying that incremental change isn't good, but that:

1. Incremental change is simply too little, too late, in a modern late-stage Capitalist dystopia

2. Because the course of politics in modern first world Capitalist counties like the US follows whatever the interests of large Capitalists are, any meaningful reform will be hindered or even reversed unless the system is overthrown in its entirety.

The second claim, that Biden doing genocide is bad and voting for Biden is voting for continued genocide, is built off of the prior point. Because voting for a right winger like Biden or a fascist like Trump will both result in more genocide, their conclusion is that voting for either is to continue genocide, though it remains implicit that if Biden stopped the genocide, they would vote for him.

I of course believe it would get worse under Trump, so as I already mentioned, I will vote for Biden. However, I also understand that protesting against Biden is the best way to change his course now, rather than later.

The final disagreement you have with these people is the idea that Biden is a "slow good" rather than a "slow evil." You're not talking to liberals, you're talking to leftists, who wish to see some form of Socialism take place in America. Biden is continuing the Imperialist project of American Liberalism at the expense of Workers both inside and outside of the US, you can't convince leftists that Biden is good, actually.

The truly best way to get leftists to vote for Biden is to get them to see what is directly more beneficial to the international Proletariat, protest voting for a third party or picking Biden and trying to use that time to organize on the ground, which is easier than under Trump. That's the real key, not to try to convince them Biden is good but slow.