With multiple layers of paint to remove I'd go with the angle grinder and 60 or 45 grit disks to remove the top layers then switch to 80 grit for the last bit. Once I saw metal peeking through I'd stop, THEN I'd switch to a DA and 80 grit.
For stripping paint a circular as opposed to a DA sander works much faster. Save the DA for when you don't want to remove as much as fast.
Even with 60 grit grinding/sanding disks it's going to take a good long time to reach the primer coat. The 3m stripper disks work great but can wear out quicker than the disks so I save them for the last bit... when I don't want to gouge the metal with the 60 or 45 grit.
Also grinders work best when using just the outer 1/3 of the disk on an angle with the spin direction working away as opposed to towards your 'target', so round or flat surface doesn't make much of a difference. A good respirator sure will though.
