@BenLin 

Hi, here is some basic information for your reference:

Most internet service providers have IPV4 and IPV6 enabled at the same time so even some old devices did not work with IPV6 and they could still get IPV4 for internet access.

Will all Deco's be rebooting?

when you enabled IPV6 on the Deco, Deco will not reboot.

 

How to tell which devices run IPV6 and which ones don't?

You could tell from the IP addresses on the clients themselves.

 

Do you see speed gain across the board or only those that supports IPV6?

I will leave it to other users.

 

How about stability?

I think it would be as stable as IPV4.

Besides the above information, I noticed your firmware is 1.2.11 which is a little old since now the latest firmware has been 1.5.6.

And if possible, you could have a look at this link to update the firmware:

How to update Deco M9 plus from firmware 1.2.12 Build 20191014 Rel. 420255

Thank you very much.