But the seats are gerrymandered...what's important is the margin. 2018 was not like any other midterm election since 1974. The losses Trump faced were likely magnified by his shittiness. Trump was a drag on the ballot, even though his name wasn't physically on it. Voters associated the GOP with Trump because he is the party's standard bearer.
Ah, but nothing about these times are normal, and twice in the last twenty years, the President's party gained seats, despite that happening only 1 other time since 1900, if your facts are correct. But to lose a midterm by 10 million votes and nearly 10% is historically unmatched since 1974. So there's a reason why that happened, and it wasn't just because of conventional politics.
Seats are gerrymandered, so you're not actually capturing the electorate when you look solely at gerrymandered seat totals, and you certainly can't divine the reasons for why the vote margin was so large by ignoring it. 2018 was a bellweather. It's why 19 Republicans have quit since. Unlike other midterms, voter engagement, and even voter turnout, was at historical highs for midterms. Hell, the turnout in 2018 was 50%. In 2016, it was 55%.