Does he get to take over my previous body too? That would be my biggest concern lol

lmao let's assume you are cloned (the copy stays in your body, unaware of the whole thing) and Joe's persona is put in offline storage

Afterwards do we remember what both bodies did, just Joe's, or just our own?

There is no afterwards, you remain in Joe Biden's body for the rest of his (your) life. Joe Biden's persona is never embodied again. You retain your prior memories, so in principle you can, for example, issue an executive order to benefit the version of you that remains in your body.

Aw shit, I misread the description, this is a vastly worse deal than I thought lol

Do I still get dementia?

You most certainly do

How much of the personality, views, knowledge, and memories is still ours?

A perfect copy of your brain and upper part of the spinal cord is placed in Joe's body in a perfect surgery that does not leave any marks whatsoever. It is adapted so that Joe's body accepts it without any side effects. Then the brain is instantly aged and damaged in the same way that Biden's brain has been damaged by aging and any other factors.