Have I ruined my CP1515n printer with sticky label stock?

Sadly yes, this usually trashes the fuser :(. The problem with yours is the fuser is tightly integrated, requiring a teardown to replace it. The other problem is the fuser will probably cost more to replace then it’ll cost to find a used CP1515 with a descent page count you can use your existing toners in.

I would either upgrade to one of the Dynamic Security printers*, match what you have or find a higher end unit from 2014 or older that doesn’t have an insane existing page count (75k+, especially used with 100k+ pages). Now yes, 100k pages on a machine built to take it isn’t always a big deal, but they’re risky at that point. They often need pickup rollers (usually ~$25 USD to do both Tray 1 and 2 on dual tray printers) or catch up maintenace like fusers (usually considered “consumable” on giant machines), as well as the ITB on the large units; sometimes drums on smaller ones which do not have the space to integrate the imaging hardware into one. The problem isn’t things like the fuser (as long as you can find them reasonably easily and the swap is easy) or rollers; on color models, it’s often the (sometimes tired) ITB. If it’s a “lifetime” ITB, be prepared to scrap it! Not fun to replace, nor cheap. Gotta strip down the unit down as well.
I don’t recommend “high mileage” machines to most people, including the ones which are notoriously known to make it like the Pro 400 series only because of the repairs needed on the rollers and separation pads, as well as other parts which can suddenly fail like the fuser. If you’re familiar with the model and what goes, they can be had for pennies to the dollar but for most people they’re a bad idea. 125k pages on a Pro 400 doesn’t scare me, BUT I know the risk and bid accordingly!!!
*These can use non-HP toner, but you’re reusing an original chip or getting toner with reused chips. These do block 3rd party toner with cloned chips.