I have not been able to hunt deer for about 5 years now because of my work situation ( long hours and very little time off
). I live in a shotgun-only area as I presume you do as well.
Back when I was able to hunt deer, I eventually selected an H&R single shot with a fully rifled barrel that was designed around sabot slugs. When I found I got excellent accuracy from Remington solid copper slugs, I began to use them exclusively. A favorite gun shop had an end-of-the-season sale one year and I bought a life-time supply. With any decent shot placement at all, these will drop a deer in its tracks.
Having said all that, I was intrigued by Winchester's recent 20 gauge slugs that (they claim) offer almost rifle-like performance. In your shoes, I would sure investigate what similar products they have in 12 gauge, or even look into the purchase of a new or used 20 (if of course you have the money for another gun).