When docking the tablet there is a preset charging limit.. any way to change or override the limit?

That is intended for prolonged battery life

10 days later

You can practically bypass this behavior on GrapheneOS (and stock Google Android, I think) by turning off the Pixel Tablet to charge it to 100%.

(In stock Google Android, there is a setting that allows users to temporarily override this, and charge to 100%; but this must be done each time the user wants this to happen. In GOS, this option doesn't exist - the last time I checked - in the Settings app; probably due to the lack of privileged Play Services installed for Pixel Tablet specific OS features.)

This method works with both the official Google 30W charger and the included Pixel Tablet stand base.