multiple times a month I’d say it just randomly charges to 100% regardless

That’s intentional on Pixel phones - from support.google.com/pixelphone/answer/7106961

To keep its battery capacity readings accurate, your Pixel needs to fully charge at every 10th cycle. This applies even if you have the “Limit to 80%” option turned on. To finish the process, leave your phone plugged in for at least 30 minutes after the screen shows 100%.