Rex, this is a good topic and thank you for bringing it up.
I think a lot of reloaders eventually settle on some "standard" loads. Some of us may have a few more standard loads per caliber than others but I believe the overall trend for most is to reduce the number of different loads on hand, not increase.
It seems a little counterintuitive at first because reloading gives you lots of options for cartridges, but the reality that catches up with you is that you gravitate towards certain loads and leave others behind.
I'm a firm believer that if I need a magnum cartridge, I will select a magnum cartridge; consequently I do not hot rod non-magnums in an effort to make them into magnums.
After many, many years of reloading and shooting, I've settled on my "Standard" loads. In fact, I've printed the data for those loads and posted it on the wall beside my bench.
Your criteria of one load for 90% of your needs with a magnum revolver (a S&W 686 in your case) may be a little too restrictive. Instead of trying to find one load in the middle, you may find two loads to be more useful. A magnum load of whatever power level you feel suits you needs and 38 Special load that fills the role of Special. There seems to be a perception that a 38 Special load is somehow weak; it is not weak. The 38 Special doesn't need to be loaded "hot" to be effective. It's a fantastic and very capable cartridge. Because the .357 magnum overshadowed it so much, people started thinking it is inferior. The 38 Special was THE cartridge for decades and it's very capable.
I don't always shoot magnum cartridges but when I do .....I choose Dos Equis - NO.....JUST KIDDING !
When I shoot magnums, I go full power. Some max load of H-110 in 357 or 44 mag. The rest of the time, one of my Standard 38 Specials will get the job done.
I understand your desire to have 1 load but with a magnum revolver, you can really have: One 38 Special load and One 357 magnum load.