My first centerfire handgun I got for myself was a Ruger Blackhawk in .357 mag. That was in the mid 1970's. Being on a budget,, I snagged all the brass I could find,, and .38 spl was PLENTIFUL! So, I loaded & shot a lot of it. And I mean a lot of it.
I've had some ammo over the decades that leaded badly,, causing more than normal cleaning. But I've never, in about 47-48 years,, with a good variety of .357's, had any damage caused by .38's in a .357 mag gun.
To address the info you've found from the internet "experts" out there.
There are people who've abused firearms in many ways. As noted above,, lots & lots of excessively hot ammo in a .38 Spl case,, when fired in a .357 MIGHT cause chamber erosion a little. And as noted above,, it MIGHT be the type of firearm metal that experiences fatigue vs a properly heat treated quality metal. But in all my decades,, & hundreds & hundreds of .357's I've handled or shot,, I've never seen the type of damage you mention.
If the desire to shoot "hot" .38 spl ammo exists,, then do as noted above. Load the hotter stuff in .357 cases.
Now,, if you buy a Freedom Arms revolver in .454 Casull, they do not recommend you shoot .45 Colt ammo in THEIR guns,, even though others do it in different brands of firearms. FA recommends a separate cylinder for any of their guns to shoot different ammo. Why? They cut their chambers to very close tolerances, and they know that even with the top quality metal,, no EROSION will occur,, BUT,, they do know that carbon build-up can cause sticky cases or failure to load a longer case. I own a .454 Casull FA myself. And because of THEIR recommendation, I load any milder charges in .454 cases just to follow their recommendations. NOT because I'm worried about any damages.
In short, load .38's in your .357 & just clean it afterwards.