That would be an implausible amount of split-ticket voting. If Biden is toxically unpopular rather than just unpopular, and if the Trump/Republican brand was not a big vote loser, then of course the rest of the Democratic Party would be losing elections instead of winning them. And yes, turnout is lower, but special elections have historically been indicative of the type of national environment, and a large gap between them and the actual national environment is logically implausible too and there is no reason to think this is suddenly happening.
I think we can rule out the idea of a comfortable Trump win-especially in the popular vote-but another close election is the safest bet. It is a long way from the election, the OP is obsessed with the idea of the economy getting worse and this hurting Biden-I disagree and the US is projected to return to normal inflation without a recession.
There was a 10% split ticket vote between Senate and governor in Pennsylvania in 2022 and 8% in Georgia.