I'm sorry, how are the "Dems" suppressing transparency? Particularly in a state like Georgia, where it is the Republican governor and Republican Secretary of State that are responsible for running the election, and that have certified it and are satisfied that only legal votes were counted? The fact is, it is the states that are responsible for their elections and for providing that "transparency" you want -- not Democrats. In some cases that is Democrats, but it is just as often (and even more often) Republicans.
Additionally, the courts have ruled (a few times now) that the Republicans were given the transparency that the law requires; I understand you don't like it and point to affidavits, but again, the courts have not found those claims credible. Instead, the courts have found that the complaints often are based on ignorance about the law (such as when particularly were barred because there were already the allowed number of Republican observers) and that legally none of the claims showed any actual violation of election laws.
As for "proof" needed to validate Biden's votes -- that is what the various states are doing when they certify their votes. They've checked to ensure, to the best of their ability, that all votes were legally cast and that state law was followed. It isn't Biden, or Democrats, that are required to prove they are legal -- that is what the states are doing. Additionally, to claim they need to prove a lack of fraud is impossible, that is one of those negatives that cannot be proven. Instead, as the courts keep telling the various lawyers working to help Pres. Trump, it is up to them to provide the evidence that fraud occurred.
As the US Court of Appeals for the Third Circuit ruled today -- and this was a panel made up of three justices appointed by Republicans, including one by Trump, stated, “Calling an election unfair does not make it so. Charges require specific allegations and then proof. We have neither here.” They stated the “claims have no merit.”
I think you misunderstand the Democrats response here. Yes, originally they argued the claims -- until they found that it didn't matter, no one on the other side was listening. The cases being filed today, at least that I've seen, raise the same issues that were claimed a week ago -- but the evidence hasn't improved at all. Now, maybe there is something more, I've not looked at the most recent suit in Nevada, where a judge has agreed to hear the Trump campaign's case.
What you miss, though, what most "on the left" argued against keep being brought up over, and over, and over. They are tired of debunking the same points over and over. Instead, what they see now are rulings, such as the above Appeals Court ruling. If you understand "legalese", the above ruling is the equivalent of calling Trump's lawyers idiots and/or stupid. The appeals court felt the need to explain basic law to the Trump campaign -- and again, remember one of these justices was appointed by Trump and all were Republicans -- by stating that, "Charges require specific allegations and then proof." This is why "leftists" are laughing, because that is essentially what the court is doing, laughing these cases out of court. And, yes, this lawsuit is now being appealed to the Supreme Court but I can't see it getting any more consideration that it did in the lower courts.