On

turning it off doesn’t add any advantages but disadvantages

ON, with the note that there are still old fashioned providers that DO NOT support IPv6. You should leave yours if they don't.

IPv6 off on my router, otherwise my Google Assistant devices don't work properly, even though my connections are only IPv4.    

Depends... My ISP only support "IPv6 6rd" today and not native IPv6, so everything becomes slower with higher latency since all traffic goes through a special bridge. I also feel like I have more control when using IPv4 with my current firewall/IP-blocklists/filtering solutions.

Also think about that IPv6 don't use NAT, so all your devices gets a public IP and are visible to the Internet. My Google/Nest speakers also got funky as mentioned above with IPv6 for me (could be related to mDNS/multicast).

I don't turn on stuff if I don't really need it and everything works great, but that's my approach. :-) Do some speed/latency-tests to see if you benefit from using it...