Is your second router configured as a bridge or a separate subnet?
If it is a separate subnet I don’t think there is any way to properly support IPv6 for the devices connected to the second/third router.

Reason being that AFAIK, you only get a /64 IPv6 subnet Prefix from the ISP for your 1st router, which is not supposed to be subnetted any further.

I don’t know if any of the ISPs will allocate something like a /56 IPv6 prefix to home fiber users. Actually I’m interested in the answer to this question myself. I planned to move to a new place soon, so I didn’t bother to check with my current home fiber provider.

See https://serverfault.com/questions/714890/ip...-work-around-it

This post has been edited by tcwan: Apr 15 2020, 12:54 PM

Sorry I have no experience with mesh routers, to know what is their network configuration.
If it can be set to bridge mode, then it is possible to have everything as one big subnet.
Otherwise you will need to get a /56 or something similar (other than using static IPv6 addresses and subnetting the /64 but as mentioned in the article I linked, it is not advisable).