I don't think the Senate will be under Democrat control. Possibly but that assumes they either retain all 13 of their seats currently up for reelection plus win 4 of the 22 of the Republican seats.
Of 35 seats total up for reelection this year, Democrats need to win 17. I don't think that is in the cards. At least not at the moment.
Democrats cannot remove the President under the 25th Amendment. That's not how it works. Nor would they have the majority required in the Senate to convict and remove him should he be impeached. They would need to win an additional 16 Senate seats to get the majority required. That, or blackmail enough Republicans to go along. Neither is happening.