Jagged edges on the model after baking

This is typical. Because texture maps are a square grid of square pixels, if you have a curved UV island (or even a UV island with a straight border but it’s slanted), it crosses through that square grid, and it’s pretty much impossible you’ll ever have a curve that crosses through the square pixel grid with the exact same number of pixels in the exact same proportions on either side of the curve. Even if it does, those pixels will always be at an angle to the orientation of the polygon’s edge.

If you need really clean lines in your bakes or textures, it’s often good practice to straighten the borders of your UV islands, and sometimes the internal edges as well, depending what you’re trying to do. There are always exceptions and it’s always situation dependent; for example if you were making something organic like character or animal anatomy, it would probably be more important that your UV shells were a shape that had little distortion that was easy to texture. But for mechanical or hard-edged stuff that’s supposed to look neat with crisp lines, it’s often a good idea to straighten the UV island. Hope this helps!