I think you're wrong in this assessment. The 2018 election cycle was an absolute disaster for the Republicans even in the Senate races.
Yes, the GOP picked up two seats in 2018. But that was an election cycle when they should have picked up more. There were 24 Senate seats held by Democrats contested in 2018, and only 9 held by Republicans. Look at all the states that Trump won in 2016 where the Democrat won a Senate election in 2018:
Arizona -- which turned out to be a complete disaster because the governor promptly appointed the loser (Martha McSally) to fill John McCain's old seat until 2020, when she suffered the ignominious distinction of losing BOTH Arizona Senate seats for the GOP
Michigan
Montana (how the hell does Montana NOT vote for a Republican?)
Ohio
Pennsylvania (GOP candidate Lou Barletta was Trump's biggest supporter in 2016, and he got his ass kicked in 2018)
West Virginia
Wisconsin
As I see it, there is far too much emphasis being put on the disastrous Senate elections in Georgia this week. The Republican Party effectively lost the Senate in 2018 when they didn't capitalize on an opportunity to make more gains in a year when they had so few seats to defend. It just took two years for this to materialize.
You are ignoring that California for the first time legalized ballot harvesting in 2018. That caught the GOP flat footed and they lost a number of otherwise safe House seats. This cycle the GOP ballot harvested as well, hence all the screaming of foul by the Cali RATS. One reason we picked up seats is that we won many of those California seats back because the GOP followed the RATS example. Montana has always been bipolar like North Carolina. We elect all Statewide conservative GOP and put a RINO in the Senate and RAT in the Governors Mansion. Montana elected a Republican to the Governors Mansion.
You have to do a deeper dive on your numbers. Trumps coattails in 2020 were very long. We lost the Senate thanks mostly to the stupidity of Georgia RINOS.