Republicans are better off with Sinema as a Democrat.
You're correct about Manchin being an outlier -- a Democratic Senator in a highly red state. If he were to retire (or switch parties), no Democrat would win in West Virginia for a long time. Trump won there 68-29 in 2020. So there's nothing to be gained on the Republican side by having Manchin switch his listed party affiliation.
Sinema is a different story. Arizona is now a purple state, and Sinema was proof that Democratic Senators can win the general election there. If Sinema were to switch parties, she would have to face a Democrat in the general election, and perhaps lose. If she stays as a Democrat, the chance of knocking her out in the primary is very small, so oddly Democrats would have to root for a Republican challenger beating her in the general election, and then subsequently losing to a different Democrat in a future election. That's part of the reason they're so furious at her, and not quite as much at Manchin.
Also, they expected Manchin to be a conservative Democrat who could be a pain in the ass, but they thought Sinema would be a moderate-but-fairly-solid Dem. She's been far more Manchin-like than anyone expected, and that pisses them off, because they know they're stuck with her for awhile, and the Senate is so close. Democrats had this fantasy that they could pass a lot of legislation with all three houses, provided they could either get Manchin to go along or get one moderate Republican to vote with them. Instead, they're having to fight TWO moderates in their own party, and it just brings down all of the major (and crazy) game-changing legislation they want to pass.
Right now the new tactic is to keep putting out the threat that they're going to primary her out if she doesn't shape up, but I just don't see it happening. Arizona Democrats aren't New England Democrats. They tend to be moderate themselves, and while supposedly 1/3 of the AZ Dems view her negatively, that's not going to be enough to push her out. In general, short of a big scandal (which she's unlikely to have), voters just don't like primarying out the incumbent. Also, her moderation will probably make it fairly easy to win the general elections, given that a lot of Republicans view her favorably right now. Hell, if I were in AZ, she'd be the second Democrat I'd ever vote for -- the other being Jerry Brown in his last gubernatorial term, but that was moot because he was winning anyway. I gave Brown my vote because I felt he earned it by being his own man and frequently telling the CA Dems to eat shit with a lot of dumb stuff they wanted to do.