What you say sounds reasonable. And, yes, GOP'ers still have an aversion to mail-in voting (I certainly do). And as I said, PA is mail-in only...no in-person EV.
Still, in general, a larger window to vote should lead to more total votes--even if only nominal. And even just a nominal increase for GOP is not ideal for Dems.
There is another poll I often cite when talking demographics, and that is the long running Gallup poll on political ideology (Conversative vs. Liberal vs. Independent). Conservatives and Moderates are roughly split at 36 and 37% of the country. Libs makeup 25%. My takeaway from this, getting a greater percentage nationwide voting is an advantage for GOP. To your point, Dems get-out-the-vote in dense urban areas that go 85-90% Dems is the only reason Dems every win a nation-wide election. In an admittedly very shallow interpretation, Dems should never win a nation-wide election.