If we somehow got 55 Senate seats but Trump won re-election this year, the silver lining is we'd probably enjoy a midterm bounce in a much better Senate map. Here's what we'd be looking at in terms of potential pickups:
Arizona - actually assuming we'd hold this by now, since Kelly winning is one of the lower-hanging fruit. If we won 55 Senate seats we'd have to have put Arizona away.
Florida - I know the state disappointed in 2018, but it's still a naturally competitive state. Nonzero chance Rubio retires anyway, since he wanted to in 2016.
Georgia - refer back to Arizona, it's not as obviously flippable but if we got to 55, it probably means we won Georgia's special election (and probably their regular election, too).
Iowa - Grassley is old as shit and thus would likely retire, and Democrats won the House vote in 2018.
Missouri - probably a stretch, but maybe Kander would be willing to run, especially if Blunt retires. Kander lost by 3 in 2016 while sharing the ticket with Clinton, whereas McCaskill lost by 6 in a much better environment for Democrats, so I think you could make the case that Kander could win an open seat during a Trump six-year-itch, even with Missouri being as far gone as it is. Still doubtful.
North Carolina - red-leaning swing state, possibly open seat (I believe Burr said he'd retire at the end of this term). Would definitely be a tier two pickup right behind Pennsylvania and Wisconsin.
Ohio - like Missouri, probably a stretch, especially since Portman has Kasich-esque, fake moderate popularity in a state that's moving right. Still worth keeping on the board, IMO - Brown is obviously a rare breed, but he still won, as did two Supreme Court candidates in Republican-held seats. Still, losing the gubernatorial race was probably a sign that Ohio won't be winnable in a normal environment anytime soon.
Pennsylvania - an obvious flip opportunity. Toomey isn't very popular as far as I'm aware, and the state went hard left in 2018. I think it's worth targeting even if Democrats win the presidency.
Wisconsin - pretty much the above, but with the added benefit that Johnson also made a "I'll retire after two terms" promise that would leave the seat open if he sticks to it. Evers' close win gave a lot of folks pause about Wisconsin, but it's important to remember that Democrats won the generic House vote by a much wider margin, as did Baldwin in winning re-election.
...and that's probably it. In a 2022 midterm under President Trump, I could see Democrats flipping FL-IA-NC-PA-WI at most. MO-OH are just a bridge too far, and if we're assuming we hold 55 seats it means we're probably playing defense in AZ-GA. That gets us to 60, still 7 short of removing Trump. Maybe Democrats could convince Trump to sign statehood for DC and Puerto Rico which would give us four extra Senators, but that still leaves us at 64, and that would raise the 2/3rds threshold anyway (if I'm not mistaken, to 69 - nice).