A) 2018 is not 2020. 2020 doesn't have a women-wave as 2018 had.
Republicans didn’t suffer devastating losses in 2018 because Democrats nominated many female candidates.
B) Republicans did in fact really well in certain swing states in 2018.
"Really well" is a serious exaggeration, especially given how poorly they did in likely tipping-point states (AZ/WI/MI/PA). Besides, the environment is even more unfavorable for the GOP today than two years ago.
Also, Biden/Harris are lacking a convincing slogan or connection to voters that would win the election for them, like Obama's slogan did. Just "return to normal" might not cut it for them.
"Return to normal" has been working pretty well for Democrats in elections held during Trump's presidency + slogans don’t cause as large shifts in voter support as you seem to think they do.
Evers barely won, Garcia got routed worse than notorious Ben Jealous, and even Whitlary was a bit less than expected.
And on the Senate side, Stabenow seriously underperformed.
And that's to say nothing of Florida, even if we accept Missouri and Indiana are just "too red" now.