So how should we redraw state lines so that presidential endorsements don't affect gerrymandered senate races?

Redrawing state lines to affect the makeup of the Senate is definitely something that's been considered. DC statehood is an example, so is the proposal to split California into two states. (Then there are less serious proposals, like New York City seceding from New York State.)

Trust me, if there's a plan to make "your guy" win, someone has seriously considered it.

It is also worth noting that redrawing lines like this do have some real benefits beyond just making a certain party have more representation. A lot of people live in DC, and it's pretty unfair that they don't have voting power in the federal government. The only reason they don't get representation is that 50% of people don't agree with the way they vote, and you need more than 50% to admit a state. It is very awkward.

There’s actually an incredibly simple solution: allow DC residents to vote as part of Virginia or Maryland, for the purposes of congressional races. There, problem solved.

State lines? No. But there are many ongoing efforts to fight gerrymandering if you are interested.

However, that is one piece. I'm not sure how effective that would be in the short/medium term.