I'm at level 103 and there no quest left besides the endless minuetman quest. Has anyone started a new game and is it any fun going through it the 2nd time. and will my last save from this play through still be here when DLC's come out?

Go for the other endings. Either with a new character or a previous save.

But will it mess up my saves if I create a new play through?

Nope, you have separate profiles for each character and then you have your saves you make as well.

Quick saves and (I think) auto saves get written over. Manual saves don't, so those should be safe. At least, that's how it works on PC, but I'd be surprised if it were different on console.

I tend to keep any savegames that I want to go back to later on a usb flashdrive.
That way I am sure they will not get overwritten.

Consoles (at least ps4) work the exact same way