The Speer Gold Dot 38 spl+p 125gr CDHP is safe to use in your 642, and Speer in general is a high quality manufacturer, particularly in their defensive Gold Dot line.
The only issue with the Speer Gold Dot 38 spl+p 125gr CDHP is that it's unlikely to perform optimally in the 642's short barrel; you're less apt to achieve expansion with it in the target compared to the 135gr short barrel load, which is designed to better achieve expansion fired from a snubby.
As bullet placement and penetration are ultimately significantly more important than expansion for adequate self defense, this isn't a major problem, but worth noting.
The most tested and/or popular current loads for J-frame snubbies are:
- Corbon DPX 110gr HP .38 Special +P
- Speer Gold Dot Short Barrel 135gr HP .38 Special +P
- Buffalo Bore 158gr LSWCHP .38 Special standard pressure or +P (the +P version is closer to a mid-power .357, so watch your wrist).
Honorable mention:
- Hornady Critical Defense 110gr HP .38 Special standard pressure or +P
- Remington, Federal or Winchester's version of the FBI load
Can't imagine what 500 rounds of WWB cost in the current climate, but good on you for getting it and planning to practice extensively. That and a quality defensive handgun course are your best investments should you ever need to use your weapon.
ETA: Welcome to the forum and welcome to J-frame snubbies -- both are great. Don't mind some of our cantankerous old war horses here -- they're generally good folk and chock full of wisdom but sometimes wake up on the wrong side of the stall or forget to use smilies when and if they're being sarcastic.