> The most effective single thing to promote a multiparty system is to switch to ranked-choice or approval voting (if staying with single-member districts) or to switch to multi-member districts with some kind of proportional representation.

The single best is to switch to multimember proportional for the legislature (which can remain candidate centric using, say, 5 member districts and STV), and that gets even better (though procedurally more difficult to adopt in the US) if you were to switch the Presidential election from a single winner two-seat President and Vice President to ranked choice two-sequential winner system (e.g., IRV or Bucklin, but after you pick a winner, eliminate that candidate, and tally again without them for a second winner as VP) where each party is structurally incentivized to compete for both spots. These not only make more parties viable they also each offer more election-day choice among candidates of the same party, denying incumbents of favored parties an uncontested sinecure.