the time in which Star Trek takes place in wasn't established until rather late in the series [..] the earliest reference, if I'm not mistaken, was regarding Trelane's time-delayed observation of Earth via telescope
True - if you define "rather late" as the second episode after Trelane's telescope incident in airing order, or third in production order.
From that episode on, they kept to the "we're 200 years in the future, give or take" version for whatever reason.
One actually wonders if the intent in "Squire of Gothos" ever was to have our heroes 400-500 years in advance of us. A writer or a proofreader could easily think "they are 900 lightyears away, which sounds distant enough; light travels that distance in 900 years; a clever way to show Trelane is out of date, and the audience will get it after a brief explanation - I mean, I did, too!" without ever stopping to think what this meant in terms of fixing the "onscreen now". Or a proofreader could think "500 lightyears doesn't sound like much - let's increase it to 900. Oh, and there's another 500, let's fix that one, too".
Of course, in retrospect, this is the easiest TOS inconsistency of them all to explain away. After all, Trelane had a mobile planet...