I can't get IPv6 in any worthwhile form from my ISP. IMHO IPv6 isn't any more useful than IPv4 if you only have ULA. And NAT is not as well supported since it wasn't intended to even be really necessary for example. So even if you are starting from scratch or just using it internally, there are some disadvantages to implementing it over just sticking with IPv6. But if your ISP actually provides IPv6 it might be worth it as long as your devices all support it. But otherwise you're going to need to set up IPv4 in addition, anyway, so you're just going to create problems for no good reason, IMHO.

Don't use ULA, those are non internet routable addresses so they will never use v6 for internet things. Use the range assigned from your ISP.
SLAAC. Because Android has one ass of a dev who refuses to include DHCPv6

Blocked by my ISP. So I have it all blocked.

I don't use IPv6 on my lab. They been screaming to the bleachers since like 2010 that IPv6 is right around the corner due to lack of addresses, and I've still seen no real reason to want to adopt for it.

My current provider doesn't even support it... so why should I?

My setup is a pile of kludges built on top of each other over the last two decades.

I started with ULAs distributed through DHCP, connected to named, which allows hosts do declare their own name and let me access local services as though I had a real domain.

My ISP eventually started supporting IPV6, but only assigned /128, so the ULAs got NAT-6ed out to the real world.

I eventually learned how to request prefix delegation from the ISP and set up SLAAC.

So now, my PIv6 clients have a) their link-local address, b) the ULA, c) a "privacy" SLAAC, and d) a unique SLAAC. All my internal services still refer to the ULAs.

I don't think I'd recommend this system for someone setting up from scratch. The easiest thing would be to go with SLAAC, if you can get prefix delegation, and set your DNS/pihole to send the unique-SLAAC address of any servers you run.

I have that conf: /etc/sysctl.d/01-ipv6.conf

net.ipv6.conf.all.disable_ipv6 = 1
net.ipv6.conf.default.disable_ipv6 = 1
net.ipv6.conf.lo.disable_ipv6 = 1

But that falls under your exception. It seems to me that IPv6 causes more problems than it solves.

I use IPv6 local only. Everything else is IPv4. Reason being, my commercial VPN does not support IPv6 and I have reservations about leakage. My ISP already ships with IPv4 & IPv6.