Your on the right track,
I like the 1 1/2 inch alum oxide disks 80 or 120 grit depending on how thick your sections. 3M has some super tough scotch brite pads that remove very little surface also.
Make sure that you not only prep your edge tops but also the faces (sheared area) as sometimes a dull shear will leave rough edges that pick up contaminants.
The 2xxx group of aluminum alloys contain copper additions and are age hardenable. As heat is applied, the copper precipitates will increase the hardness and strength of the base metal. This age-hardening can also occur at room temnperature over long periods of time. 2219 contains about 6% copper and this is probably the source of your indications down the center of the weld and previous cracking problems. We used to use carbide burring bits and wheels with a coarse cut for aluminum to prepare welds with success. Wire wheels, flapper wheels and sanding discs tend to smear the surface and capture oxides under the smeared metal unless only light pressure is used. ER4145 is recommended for best success on 2219, but ER4043 and ER2319 can also be used.
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