Have pretty much done it with the tape like you have above, one side of the line at a time.
My trick is I generally use a 6" wide steel blade with the handle cut off for applying the mud.
Then I use a 5' long x 2" wide metal ruler with the green tape on it to keep the bondo from sticking to the ruler. Then I use the long ruler to screat it straight from the tape line up. The trick is to apply the bondo as straight as you can possibly get it, makes the sanding process a lot easier. Then sand that back straight with the idiot stick, long and skinny 36 grit sand paper.
Then do the same for below the tape line.
May take a couple of times for each side of the ridge as you are alternating top to bottom, building up the low spots on the ridge to keep it straight.
Then block sand it again with the tape top then bottom, 80 grit then down to 180 grit.
Then can do the same block sanding with your primer surfacer, the longer sanding blocks work good for this as you are bridging from the bondo work to the bare steel. Being careful not to undercut the bondo to steel area, as I call this the lake effect, the undercut area looks like a lake when you spray the wet primer surfacer on it.
Blocking out the primer surfacer is definatly part of the straightening process. They have the finer grades of finishing bondo that can be used over the primer surfacer for any imperfections or low spots that show up as you are bringing things up to straight grade.
Then block sand out your final coats of primer surfacer as you are getting it ready for applying the paint.
Here is one I have done with the above process, even that fancy curve just over the wheel opening.
View attachment 1715591113
Straightening process to the quarter panel below:
View attachment 1715591114
