Crimp spade connectors in breaker box?
You are correct in addressing this, but the ring terminal isn't the right tool for the job
You are indeed correct that 14AWG is well outside the listed/labeled range of the lugs on a 50A Siemens QP breaker -- they're rated for 8AWG through 4AWG wire only.  However, nobody's tested them with ring tongue terminals either, so your ring terminal solution is off base.  What I'd do instead is use a short length of 8AWG stranded THHN and an appropriate wirenut as a transition pigtail to adapt the 14AWG leads on the CHSPT2ULTRA to a wire size your breakers are rated to accept.  (If you insist on doing this using crimped connections, you can use a suitable closed-end crimp splice instead of the wirenut, as doing the transition with a butt/in-line splice requires a special reducing splice connector, the Panduit ESV8-ESV14-Q, that isn't a hardware store item.)