Amana makes “Carbide Tipped Straight Plunge” bits for wood with 1/4" shanks and cutting diameters up to 1". The 1/2" version is in Vectric tool library, so they likely have it in their Fusion 360 library too. If your spindle supports 1/2" shanks, you should use them. Either way, you should minimize stickout and maximize speed (RPM). @fenrus has tried the 1/2" version - you might want to contact him for his impressions (and share yours if you try one.) FYI @Vince.Fab says that Shapeokos do better with shallow wide cuts than deep narrow cuts.
You might want to monitor and log the spindle input power and/or current to make sure you’re not pushing it too hard. You might find this useful too. 
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