Hi dusatidusan! 😎
You've asked two separate questions here in the header and text.
I think you also need to consider the caveats with and without a pick.
How do I strum? Both.
How should "we" strum? Hmmm....
IMV, like most things in life, acceding to any dogmatic perspective is a self imposed 'prison'. As the ancient Greek philosophers were wont to say, paraphrased, "take the two extremes of any position and the truth lies somewhere in the middle".
In my journey so far, until provided with irrefutable logic or evidence to the contrary, my personal practical experience has illustrated that both are useful 'tools'.
The practical litmus tests on my journey.
With a pick.
Try playing the rhythm strum of "Lyin' Eyes" consistently mezzo forte at its tempo of 133BPM for seven contiguous minutes without strumming from the elbow and see A. how you go and B. how it sounds. 🙄
OTOH, playing "Hallalujeh" solo acoustically piano at its much slower tempo, from the wrist can be useful for greater control over the individual notes and or tone, or if preferred to play it that way, for partial or semi arpeggiation. But it works strummed from the elbow too depending on interpretive presentation.
So as an overall generalisation with a pick, which I prefer, ordinarily from the elbow, with the versatility caveat to adapt as required as mentioned in my immediately preceding paragraph.
Without a pick.
By self-observation, almost invariably from the wrist. But, with the caveat I wouldn't strum without a pick for an amplified acoustic rhythm part performances for instance of "Lyin' Eyes" (Eagles) or "Don't Get Me Wrong" (Pretenders).
"Right" or "wrong" according to dogma I neither know or care. That's how I strum for the genres I currently play. If I ever head to i.e. Classical on nylon, I may accede to the conventions of how to hold the guitar traditionally and how to strum for flamenco etc. Always willing to try a different technique if it is proven or find works better in any specific scenario.
