Bootloop - can get to Fastboot, but volume buttons just restart the phone

Pixel 3 XL - has worked like a champ for years, but just started bootlooping. If I hold vol down + power, I can get into fastboot. However, volume down and volume up buttons don't let me navigate. Either one of those just restarts the phone. I don't believe there is any physical issue with the volume buttons since they've always worked perfet. If I let the phone just sit in FASTBOOT mode, after about 5 seconds it restarts and starts looping again. I needed a phone right away, so I found another used Pixel 3 XL, and I'm using that now. I'd love to get the old one booting again so I can sell it, or at the very least, ensure that I do a factory reset on it before bringing to a recycler. Any ideas how I can get out of FASTBOOT mode or securely delete my data?