Just as I move my domain around between email (and other) providers, I could then move my IPv6 block around. Instead of asking Linode for an IP address out of their block, I could tell them "serve this block." (Or can we already do that? For a reasonable price?)
If you control the DNS there's no real need for owning your own IP addresses as you can just change what they point to. That's usually also much faster to do in a pinch than find a new party that will announce your block and convince the old one to stop doing so. That's usually hours if not days in lead time.
Though if you truly care enough about it you'll also want your own DNS servers and own gTLD in order to own the whole chain.
However what you describe -- building out your own infrastructure -- still remains a lofty but worthy endeavor for anyone with the skills and determination.
Businesses of all sizes rent stable IP addresses. Some ISPs in some countries also provide them to non-business subscribers as part of the standard subscription.
It's really not too difficult to have a persistent IP address for an individual who only needs reachability and is not trying to run a high traffic, global enterprise. Avoid the popular large, high-volume providers. Obviously do not use something like AWS.
Having a custom domain would get your far enough in this case, owning your own IP address isn't really needed.