Also I would say that bosses don't really have that much indeterminancy in ER. At best they may have a couple of combo routes and respond to the player healing or casting a spell, but they are very predictible and repetitive.
They are predictable in the same way perilous attacks are predictable in Sekiro. Which is to say, yes, very, but also not really. I can't count the times in Sekiro i did a Mikiri instead of a jump stomp, and a jump stomp instead of a Mikiri, and i think most of you have as well.
Elden Ring for me feels very similar with the bosses having their combos being "decoupled" from any particular set structure or sequence. The fact the boss can launch into any combo at any point means you have to train yourself to be ready for each at a drop of a hat. It's not impossible to do, but it can take a long time to master, more time than some people are willing to commit. As for healing and attacking, there are determined times you can do that which i'm afraid cannot be deviated from, but generally speaking if one is possible the other is possible as well, meaning if you have a window of opportunity to deal damage you can often use that time to heal yourself. What you CANNOT do is heal whenever you want to heal, or deal damage whenever you want to deal damage. There windows of oppurtunities you have to discover which can change depending on your playstyle.
In terms of this "rythmn" game argument, it would be like you are playing dance dance revolution, except sections of the songs are broken up and can shift and play randomly at any point. You still have to train to hit the right button and the right time (it's a bit more complicated than that in Elden Ring, since there's a lot of things to consider, the posture damage, the fact heavier weapons can stagger a boss but not lighter weapons etc), but the sequence is random. So it's a rythmn game except the song is broken up in pieces and the parts all play at random following no rule.
Sekiro too of course has a lot of this as well.
The idea that you are just mindlessly perfoming a specific series of actions you can just learn by rote with your brain shut off is utter nonsense. Everything is dynamic, everything requires you to be on your feet and in many cases figuring out what you are supposed to do isn't exactly easy.