I have several Shields, but not a Shield Plus.
It wouldn't kill S&W to include a simple plastic mag loader, like the one that came with this 13+1 Springfield.
Pro tip : Loading up your mags to capacity and leaving them for a few weeks in the hopes they will be easier to load does very little.
What "will break in" a spring is repeated cycles of loading and unloading (firing).
Magazines that have been left loaded since WWII still have springs that work fine. What will eventually wear out a spring is actually using them out at the range, not leaving them compressed.
Everyone here that has had a Shield or other small pistol with hard to load magazines all report that after some use, the magazines become easier to load and the loading tool, while still nice to use, isn't as necessary anymore as it was when the mags were new.
I'm sure, like me, those guys all went out and shot a few hundred rounds thru the new pistol, rather than load up the mags and check back a few weeks or months later to see if they've eased up.
If you want to have the Cadillac with the bells and whistles, get the UpLula.
If you want something basic that will let you get that last round or two in, just get a few of the Range tray or Makershot loaders, you can buy 3 or 4 for what the Uplula costs.
That's what I did for my Shields and keep one in the range bag, one in the gun room, one in the glove box of the truck, etc.