2. to switch to the new service with one non-static IP (how can it be static when Zen can change it whenever they like).
Zen doesn't do "dynamic" IPs which allocate you an IP from a pool on the LNS each time you connect; they do assign you a static IP and you get the same one every time you connect. But yes, once in a blue moon they could decide they need to move you to a different static IP.
FWIW, I had the same happen with Cerberus: one morning, I found my IPv6 stopped working. Turned out they had unilaterally changed my assignment, without warning, for some operational reasons (i.e. they needed to stop using a legacy block and move me to a new block). They promised it wouldn't change again, and it didn't. But I'm not with them any more.
When I started on Aquiss, I initially had an intermittent problem with IPv6 dropping every few months. They recommended that I move over to their "new" network, which also involved renumbering. But this was a fully planned and scheduled change: they were able to give me the new allocation before cutover, choose a convenient date with lots of notice. They handled it really well, and the service has been rock-solid since.
In case it helps others in the same predicament, I intend to try to move to being broadband provider agnostic using VPN's to VPS, Cloudflare tunnels, and IPv6
I think that is wise. Note that IPv6 by itself doesn't give you any help when changing provider, since you must use the IPv6 block assigned by your provider out of their address space - that is, you'll have to renumber all your IPv6 when changing provider. That's unless you do IPv6 NAT with ULA addresses internally (fc00::/7) which I would not recommend.
Another option is to take the AAISP L2TP service, which gives you static IPv4+IPv6 from AAISP's range, but can be accessed through any broadband connection. But your VPN to VPS and Cloudflare will give you much of the same benefits. There is also
Pangolin, which is an open-source project similar to Cloudflare Tunnels which you could run on your VPS.