There's no ineffective 12 gauge slugs when it comes to medium sized, thin skinned game like deer.
I used 2 3/4" slugs for 4 decades with great effect. My longest shot was about 65 yards,IIRC, and there were a couple measured better in feet than yards.
From the bench, my two slug 870s can keep them around 4" at 100 yards. One's a smoothbore, the other has a rifled choke tube.
For you, test some rifled slugs and Brenekkes until you find the one that does best in YOUR shotgun. Then, using a hunting position, stick to whatever range you can stack them into a 6" circle.
If that's 20 yards at the start, so be it. As you improve, so will the ability to make a clean, humane kill at further distances.
But, best slug hunters hunt like Archers. See how close you can take them, not how far off.
Over the 90s, when I did some great slug hunting on MD's deer rich Eastern Shore, my average shot opp was about 32 yards. Out of maybe 25 or 30 deer I shot, I lost one.
HTH....