Democrats in office right now will never, never put up a fight
Aside from a few notable exceptions, I agree. That's the reason why it's important to have a clear message - "Democrats cannot be trusted to represent our interests, they are our opposition to radical change....."
Immediately following that with ".... but we must continue voting for them anyway" confuses that message. It also serves as evidence that there is no popular movement for radical change because those people advocating for it keep voting for status quo anyway. Again, it's not about how you actually cast your ballot, it's about spending all your time proselytizing about how important it is to support them anyway, even if begrudgingly. It turns your leftist principles into nothing but a performance.
The only thing that will reverse the trend within this generation will be outright revolution
You can't do either of those things by gaslighting liberals into thinking radical change isn't possible because radical change isn't even popular enough to overcome soft-power legacy media, so you must continue participating in lesser-evil politics until the revolution comes.
The civil rights act didn't get passed because liberals patiently waited until there was a critical mass of popular support - it passed because the movement and MLK specifically agitated liberals repeatedly and threatened to interfere with their political standing if they continued obstructing the change they pretended to care about. Liberals then, and liberals now, threaten the destruction of the union by obstructing that change which is being demanded.
The democratic party won’t ever put forth a leftist candidate until there are literally no other candidates for it to choose from
This is almost true, but just a little misleading - it's not the democratic party that won't allow it, it's liberals who make up the party that will selfishly obstruct radical change until their place of privilege within the existing system is materially threatened, either by the fascists they have been collaborating with or by hemorrhaging the working class they have abandoned. Any protest, direct action or """revolution""" will amount to how large of a threat that is, and if the online 'radical leftists' can't even agree on an uncommitted stance in public then those aren't really leftists at all, they're just liberals in denial.