Bit to use on landscaping timbers? 30 1/2" holes.

I don't think you'll have a problem. Building my shed, I drilled at least twenty 1/2" holes in pressure treated 2x and 4x, attaching the posts. I used a new Diablo 1/2" self feeding spade bit. If anything, it cut too fast. Cut a little, back it out to clear chips. My 18V Makita worked fine, but I switched to my 30 year old Craftsman corded drill (because I'd rather replace the Craftsman than the new Makita.) I didn't drill more than 4 holes at a time, so the drill motor never got hot. Both the drill motors and the bit still work as well as they did before the project. Tools are meant to be used.