Just from the list in my post:
> elections being on a non-holiday Tuesday
Simple majority legislation
> absentee ballots being limited / restricted and operating state-by-state
Really easy to correct with conditional budget funding on the basis of providing / enabling absentee / mail in voting, again a simple legislative majority on a budget bill
> the persistence of first past the post voting being the least representative form of election
First past the post isn't codified into the constitution. The only part that requires constitutional amendment is the electoral college but that can already be overriden with a combination of popular vote interstate compact and said statewide elections being held in an approval or score voting way.
This one requires policy change that is federal law currently mandating first past the post, but its not constitutional. Additionally this could be done in a way that removes federal simple majority requirements in federal elections and instead uses federal funding incentives to pressure states to adopt more democratic election systems because that would face a lot less blowback from anti-democracy interests.
> the continued existence of the electoral college and senate
This is the one that absolutely requires amendment. The electoral college can largely be made obsolete with the interstate compact but little can be done about how undemocratic the senate is. That is wholly intentional though - the senate was introduced to give less populated states more influence in government and to largely provide capital a numerically small wing of congress to control more easily monetarily.
> nothing to correct the loss of voting rights for felons
Felony voting rights are currently state by state, and again federal law or financial incentives could be used to pressure states to not deprive felons of their voting rights, or it could be codified as law with simple majority.
So of my stated points 4/5 can be done with just regular law, much of it done with just budgetary law requirements.