Yeah, Disney has been way behind the ball with their 4K blu ray and 4K streaming options, even though they have caught up a ton over the past year. Now, at least all the Star Wars and MCU movies are finally available in 4K digital and on 4K blu ray. As for your streaming problems for what quality level you have access to, that is exactly why I don't buy digital purchases on Itunes, but rather have focused my library based in Vudu. Its super complicated understanding all the intricacies, but a lot of it has to due with Apples agreements with the linking intermediary Movies Anywhere platform, as that is the connecting link to spread your movies out to all the different platforms. The biggest areas Apple falls short is that there are a few studios, most notably Paramount and Lionsgate that don't have agreements to link at all through Movies Anywhere, so if you buy a Vudu copy of those studios, it won't be available at all on movies anywhere, and it also won't link at all the Itunes, and vice versa. Then, there is the certain codes that will only give you 4K on Apple and not on Vudu and vice versa as well. Regardless, based on the most advantages by way of codes, and also being a superior streaming platform, I'd recommend everyone strongly consider Vudu as their base platform and I think you'll find a lot less code issues moving forward. Vudu also tends to have better sale deals on digital codes than Apple.
As for HBOMax's lack of 4K content, its becoming abundantly clear that in their rush to get their streaming platform online as fast as possible, they opted not to focus on certain things in order to focus on others. This rush has ultimately somewhat turned into a disaster for them with how many issues and complaints everyone has had with the service since it was launched. In the end, I do see HBOMax working out all these issues and having a good end product, but it will likely take a little while. In the meantime, I'm okay just waiting.