A spade bit might have a rough punch-through, so go slow and have
patience at the end point. Make sure the bit is sharp. I think I
would try welding a spade bit onto a 1/2" pipe and make a separate
chuck insert (fitted/welded onto a 1/2" sleeve, for the pipe) for
fitting into the drill chuck. A spade bit is a lot cheaper than an
auger bit for a one-time use (once welded).

If access space, in starting, is limited, also, the 1/2" pipe can be
in successive 24" - 36" sections. Any connecting sleeves will pass
through the 1-1/2" holes and the chuck insert fitting can be moved
back, as each successive pipe section is attached. Save the set-up
for future use (???, LOL) and use the pipe sections for pipe clamps.

Sonny