That’s a pretty soft temper but ive read it chips well.
First thing is workholding. You’ll want to attach a sacrificial wasteboard then surface it so its machine level. Then use the superglue/tape technique to attach the sheet, use something heavy to press it down while glue is drying. Nyccnc has a good video on this technique.
Cutting - t3 to me = soft. That means a 0.125 single flute hss or carbide will be the best bet. Shoot for 0.001 chipload, use a cutting oil or wd40 to prevent chip welding. To play it safe, cut 0.010 doc but cut into the sacrificial board so you have a clean bottom edge. You might need to cut 15 or 20thou deeper
For engraving cut lines I reccomend a two flute 60 or 90 degree Kyocera V bit. For a larger, surface style engraving youll probably want to look into diamond drag bits. You’ll want to do any engraving first before the outer contour cut.
Due to you sheet thickness, setup needs to be on point but if you follow these tips you’ll be fine, that’s how I’d run it. Keep in mind that this part wont be able to be welded but rivets will be fine. It will also have poor corrosion resistance due compared to a 6061.