Simple. End mandatory voting and kill e-vote machines.
Great. Now you have at least 6 different big changes to make, two of them to the Constitution! That's not "simple".
There's already a lot of people who don't understand how the current election system works, and many others simply don't give a fuck, so why they should be forced to vote at all?
You should never push something just because "people don't give a fuck\don't understand anyway". That's tyrannical. People need to at least in principle be able to understand the system if it is to be legitimate.
This is why electronic voting isn't a good idea. It creates a black box.
And e-vote machines can't be trusted at all, so we need to go to paper ballots anyway.
Yes, absolutely. But that's a completely different reform. People already confuse "voting system" with the voting machines. Every idea you tie together in a "package" makes it even harder to promote and pass, and more confusing to voters.
In contrast, Approval Runoff adoption here would require minor changes to the Electoral Code (effectively, "remove ONE rule") and to the machines, and it would pave the way for similar reforms in the legislative elections, which paves the way for future reforms.
About runoff: it's a bad idea. It just incentivizes hate speech.
Yes, but getting rid of it requires major changes to the Constitution AND the Electoral Code, both of which require supermajorities in both houses.
Also I don't see any major barrier on the Brazilian Constitution specifically against score voting, except for president, but this can be amended.
The Constitution has specific wording about the type of voting (simple ballots) to be used in all elections, and the winner must have a majority\plurality of votes. This is incompatible with score, so you'll need to change it.
I think there isn't much to do about bullet voting, if the voter is too dumb/idiot to insist on this tatic, well, it's her/his own fault, not the voting system fault.
I'm just saying it addresses the criticism, not that the criticism is legitimate.