Sechel, who mentioned the steipler? And yes, you don’t need to follow brisk, so please don’t mention rav chaims shitah about kavanah as a way to undermine the importance of zmanim when brisk holds very, very much of zmanei tefilah.
Re, other rebbss davening late; that is correct, and tefilas nedava is only half of the answer you’re reaching for, which is the shtikel torah of rav Meir shapiro, who said that a rebbe is chazal’s “toraso umnaso,” who is patur from tefilah. Any tefilah they perform is a tefilas nedava, which does not require the zman lf shachris, mincha or maariv.
That has nothing to do with other people besides the rebbe.