I've seriously tried, but I cannot get into to anything beyond Basic D&D and AD&D1e. D20 just does nothing for me. Every version that came out, I tried to read the game rules and be part of the interest but the game is so different than Basic/ADD1e that it might as well be an entirely different game altogether.
I really tried more so with 5e because I was under the impression that the creators made this so it would interest the Mystara people, but it doesn't. It's like playing football one day and waking up in a place where football is now called soccer and you were expected to jump in and learn the rules.
Yeah, I feel ya. It took me several years to get on board with the d20 System, and a few more to get on board with 5th Edition. I started playing D&D back in the 80s with the basic "red box" rules, and the math just feels...weird now. Not better, not worse, just different and strange. Every edition since the Basic & Expert Set has felt a bit like a pale imitation at first.
At first. After a few gaming sessions, the newer editions began to grow on me (or grate on me, depending on the edition.) None have the same BECM feel that I grew up with, but each one can be fun and balanced in its own right and after playing a couple of adventures, seeing how the math works, tinkering with the character options and dungeon design tools...well, I'm a fan. It's no BECM, but it scratches the itch.
I don't know if you are looking for suggestions, because it sounds like your mind is pretty well made up on this matter. But in case you ever decide to give 5th Edition D&D another chance, there is one simple thing that you can do to help bring back that "BECM" feel: stop using optional rules.
Multiclassing? This didn't exist in BECM, and it's optional in 5th Edition.
The battle mat? Wasn't necessary in BECM, still not necessary in 5E.
Feats? Didn't exist in BECM, don't have to exist in 5E.
Flanking, buying and selling magic items, inspiration? Not in BECM, all optional in 5E.
You can do other things as well, as others have suggested (limit the number of races and classes available to the players, trim back the number of spells, use 3d6 to roll stats, get rid of cantrips and hit dice healing, etc.) but I recommend starting with just the optional stuff at first. The math will still feel weird, but you will have gone a long way to bringing back some of that classic D&D feel.
And hey? There's nothing wrong with playing the older editions, and I promise you aren't the only person in Indiana who misses those "old school D&D" games of the 80s. I bet if you put out a few feelers at your local game shop, or reach out to your old gaming buddies on social media, you could put together a Classic D&D game night in no time at all. If I ever saw a flyer that read "Players Needed for Isle of Dread Play-thru (Classic Rules)" in my town, I'd be there with bells on and three friends in tow.