How far to grind? Depends on how good your repair is after welding. If there is some warpage and the weld isn't higher than the rest of the area by more than .060" your probably very close. Grinding to thin just for sake of appearance now could cause issues later. Use a straightedge and check for high spots. If it's low leave it.Make sure there are no pin holes or seal it from behind. Personally a cut off wheel with good hand movement will knock the weld down quicker with less surrounding damage to the sheet metal than a disc. then finish with a hard backed disc. We are hobbyist here. That means as we learn our patches will not be like someone who has done it for years every day. The longer you do it, the better you will get. I did these awhile back. I've also done some when learning that weren't as nice as the ones you pictured. You're off to a good start.
Doug