Like you said yourself, they are trying to give experienced souls players something good also, yes it might mess with your muscle memory. But isn't the whole point of souls like to be a challenge? I think they did a great job at doing it in order to give people familiar to the genre a new challenge.
And as I said in two previous posts :
1) Yes, the idea itself is good to add variety, some measure of unpredictability and force people to learn new ways to deal with foes.
2) They went overboard with Elden Ring, to the point of the game stopping to be fun to just become frustrating. As someone on Reddit said, I have the feeling that the bosses are the ones having fun in a fight, unleashing all their combo and abilities while I'm feeling restricted to dodge and run after them.
There is a rhythm, a dance in previous DS bosses. Elden Ring is so intent on baiting and tricking that there is no rhythm to the fights.
The "not fighting a powerful foe" is very debatable. Bosses like Artorias and General Radahn are characters that share very much alike and both feel extremely powerful when fighting but are both said in the games to be in their weak form.
Artorias was fun. Radahn is a pretty original fight, but horrible to play. Between the heatseeking arrows that two-shot, the ridiculous spam attacks (and the ridiculous horse-to-rider ratio, but at least that one is funny) for something so large, the usual camera gore when fighting large enemy (a long-time problem of the serie) and the instakill meteor, the neverending first phase followed by quick death and replay from the start, I really didn't find anything fun in it.
Many dark souls bosses also don't feel powerful, but it doesn't mean it takes away from the experience. Yes, i have to say the combos bosses do in elden ring can sometimes be very baity when they have a very slow windup, but it doesn't mean they are hard and frustrating. Frustration is part of the genre and is something you should be accustomed to now if you play souls like games.
Frustration, like difficulty, is a tool to reach fun. It's part of the process to feel achievement and satisfaction when you overcome the obstacle.
But Elden Ring bosses are so artificial and unfair (not all, but too many of them) that when I win, I don't feel "I managed to kill him, AHAH I SUCCEEDED !" but more "that was such bullsh*t, what a relief it's over".
Like I already said, I precisely feels that they became so famous for games being "difficult and frustrating" that they ended up making this the focus instead of just a mean.
Just like dark souls it's all about learning the patterns and if you can't add the small windup the bosses have in elden ring to it, then it's a problem for you because i am completely fine being able to do it.
To each their own in the end.
Obviously each person is entitled to their taste. But "I'm find with it so there is no problem" is a dumb argument, especially as Dark Souls are famous for having widely different experiences between people (and even widely different experience during subsequent runs). The problems I listed are not just "there are too many attacks with delays" (though there are), but a more general design philosophy that seeps into much more than just this aspect.
You're fine with it, great, doesn't mean your opinion is universal (nor is mine) nor that there isn't any valid criticisms pertaining to said design philosophy.