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You could try a laptop cooling pad/stand instead of a tablet holder. Those are meant to hold heavier devices that people type on so I'm assuming they will hold up fine. There are several models on Amazon that have adjustable heights.
Just as an example (caveat: I have not used this) https://smile.amazon.com/Halter-Adjustable-Microbead-Bolster-Cushions/dp/B01G7J5QN8/
I use a lightweight portable stand currently: https://smile.amazon.com/Steklo-Aluminium-Adjustable-Portable-Universal/dp/B00VIZ60QY/
The arms can be adjusted in angle / locked in / lengthened to facilitate working in portrait mode as well. However, it's at a fixed height (1.5"?) so not height adjustable.
Nonetheless, I'm looking for a case/folio with a built-in adjustable flip stand a la Surface Pro (no such luck yet, at least not with the same level of sturdiness) so that I don't have to carry a separate stand with me.
I am looking for one too. Someone on this sub recommended https://www.amazon.com/Halter-Adjustable-Microbead-Bolster-Cushions/dp/B01G7J5QN8/ref=pd_rhf_gw_p_img_13?_encoding=UTF8&psc=1&refRID=JYRPDH81R8KY3PPY8D2V a while back