Republican legislatures need to exert control? Um, they tried and were overruled by Democrat governors, courts, and even election boards that changed the rules as they saw fit.
Assuming Democrats don’t further stack the deck in their favor now that they control Washington DC—a very big assumption—Republicans must employ the same ballot harvesting tactics. There are more registered Democrats than Republicans, but we still have millions of Republicans who sat on the sidelines this last election. Those Republicans must be contacted and encouraged to vote—not in person but by mail as too many voters are apparently too lazy or busy to vote in person anymore.
That’s why the Democrats win mass mail in balloting. Normally, many of them wouldn’t bother voting, but when harvesters show up and make voting as easy as possible, these unmotivated voters will submit a ballot. Of course there’s also fraud—no one can know it wasn’t enough to change the 2020 election—but these mail in ballots and ballot harvesting are what’s really killing the GOP.
They didn't try. The republicans could have assembled a quorum in the conference room of a local hotel and voted and submitted their slate of electors.
You are correct on the ballot harvesting though. They got away with it because of massive mail out ballots. But if the GOP plans to win anymore elections they need to start sending volunteers to peoples home offering to help them vote. Not vote for them, just help with getting the ballots.