“The problem is this ‘raises the bar’. Calvinists must now shoulder the difficult task of proving a negative – they must specifically take out Arminianism. So instead of showing XYZ is taught in scripture (or the preponderance of evidence leans that way), which is all the Arminian must do; Calvinist must show ABC is denied in scripture (not just ‘not taught’, but explicitly denied). In short, internal disagreements within Calvinism require them to shoulder an a-symmetrical burden of proof in comparison to Arminianism.”
There’s no asymmetry here.
An Arminian must prove conditional election to the exclusion of unconditional election, while a Calvinist must do the reverse.
An Arminian must prove corporate election to the exclusion of individual election, while a Calvinist has to prove individual election (although he doesn’t have to disprove corporate election).
And that applies to other issues as well, viz. limited/unlimited atonement.