Welding Tips and Tricks

I have an old Miller 200 that has been a great machine. So my question is; If I obtain a spool gun for aluminum welding is there a limitation on aluminum thicknesses one can weld with a 200 amp machine? I have never welded MIG aluminum and don't know what to consider doable given the amps might be limited. I am concerned that the welds be structurally sound, with 100% root penetration. I have seen plenty of MIG/ aluminum welds that had zero root penetration so I am wondering if the issue is nearly all bad technique or due to poor machine set-up or machine limits being surpassed.

The miller weld calculator suggests 180-200 amps for 1/4" aluminum with 3/64-1/16 wire spray transfer. Check the spoolgun too, a lot of the cheaper ones are only rated to 150 amps at 30% duty cycle, so that might be a limitation.

Joint prep is one of the most important aspect for full penetration. Bevel and clean.