What happens if I stay in Thailand after my work permit expires but before my visa expiration?

You should be on an extension based on employment (Non-B), not a visa any more.

First of all, you misunderstood some things.

1. There is no grace period once WP and extension are cancelled unless your employer is a BOI/IEAT company. You have until 23:59 on the last day to leave Thailand without overstaying.

2. Your extension is invalid when the WP is cancelled or expired, as WP is one of the requirements for your extension based on employment.

So if your workpermit expires on the 23rd, technically you are overstaying after that date.

Q. In practice what is the real risk for staying after work permit expiry and not cancelling it?

-> You cannot apply for WP if you don't cancel it. You cannot apply for an extension if you don't cancel the current extension officially. You might be able to apply and get an initial Non-B visa, but not the following extention. You will be able to cancel them later so that you can go to another employment in Thailand, but you may be fined for not cancelling properly.

- We would leave before 29th to avoid overstays.

-> It may or may not work. Maybe the immigration officer misses the issue and stamps you out, no problem. Or catch it and give you overstay fine. Just being luck or not.

You can apply for another extension when you cancel your extension on the 23rd for whatever reason (for example, tourism), without enough documents. Then your extension will be denied, and you will get the (up to) 7-day-to-leave-kingdom stamp. 7 days will start on the day of application, so it will cover until the 29th. Don't forget to bring the booked flight information to get the stamp by the 29th if you go this way.