I was too impatient. I rewired a new board because “why not?” Works flawlessly. Definitely the other board sending bad signal. Thanks for all the input, @profezzorn.
I’m still going to potentially try the newer servos before finalizing this blaster build, but thought it helpful for other folks following the thread to know some Proffies are sending subpar signals, and to try another or get them from more reputable sources before giving up on your builds.