What's a good reduced load (factory) .357 for "regular" shooting/plinking? - so as to avoid any fouling (cylinder build up) issues - and therefore specific cleaning necessity - with shooting .38s then immediately wanting to cross-over to shooting .357s.
I hope what I'm going to say is responsive to your post... but...
... there really are no 'fouling issues' using .38spl in .357 chambers. I have a Smith 66, and two Ruger Security-Six revolvers, all .357s.
But I've always shot 95% .38spls in them. My own reloads, stuff I buy on the cheap, or LSWCHP +P FBI loads. And a few .357's when I want to give myself a thrill.
I've never never had any problem at all with fouling. I don't even clean them every time I shoot, either... they're stainless. But normal cleaning like you'd do in any revolver, no matter what you've been shooting in it will do the trick. The fouling thing is a non-issue. Truly.
A HUGE part of the utility and fun of a .357 is the ability to fire .38 spl in it. You lose that, and .357 becomes a bit questionable: expensive, and inflexible.
Also keep in mind that the ability of a .357 to fire .38 was designed into the system... it's not a 'trick' that some dodgy dude came up with in desperation... like firing .32ACP in a .32 mag revolver.
You'll solve all your problems in the most ideal way by simply rethinking your position on firing .38's through the firearm.