My 60 year old house with a detached garage has the exact same footing that you are planning on using.
This held up for 60 years however I did have some termite problems over the years and I eventually painted the entire footing and lower portion of the wall studs with "Termin" a wood preserver and anti-termite treatment.
The only thing that I would recommend (and I'm not a contractor), is that if you plan on sheetrocking, you might build up the base so that the sheetrock is not so close to the floor.
See my photo where I sheetrocked the garage. My wall studs were built on top of the 2x6 base (like yours). But when I sheetrocked, I put a 2x4 base first, which keeps the sheetrock an additional 4" further off the ground.
Lastly, using pressure treated wood is good, but if you make any saw cuts, you should treat the exposed cut with termin or similar preservative.
