I'm going to disagree with you a bit here as sorry, I know a lot of fans like to think (and I too used to think) STAR TREK was the first and only 'mature' science fiction show out there (and when you compare it to the other really popular Irwin Allan series like Lost In Space, The Time Tunnel Land of the Giants and occasionally Voyage to the Bottom of the Sea although Voyage was more an action/adventure show when it started); you'd be right.As I said before, in its day, Star Trek was closer to "hard" SF than anything else on TV. Everything else was pure fantasy and often painfully science-illiterate. Star Trek actually consulted with real scientists and engineers, and though they often chose to ignore the scientists' advice in favor of a more fanciful approach, it was always an informed choice rather than the lazy ignorance of its contemporaries and successors. I doubt you can find anything in 1960s, 1970s, or 1980s SFTV that's anywhere near as credible as Star Trek was at the time. Well, maybe the 1988 series Probe, which was co-created by Isaac Asimov, but that lasted all of 8 episodes.
Part of the reason Star Trek was so compelling to so many people when other SFTV shows fell by the wayside is that it was the only SF future on TV that felt even remotely close to plausible. So when people today dismiss it as being just as fanciful as its contemporaries, or even worse, they're misunderstanding why it was so important and special for its time. The only reason today's shows have surpassed it is because they built on the foundations it laid. In its day, nothing else even came close to its level of believability, even with all the liberties it took.
But, when you consider what original The Twilight Zone did when it occasionally branched onto a Science Fiction story; as well as the original The Outer Limits, which honestly had a number of fantastical, but seemingly plausible stories like The Galaxy Being, or The Architects of Fear.
And if you want something from the 1980ies in the U.S. - the very first V (1983) miniseries (not any of the later mini series or other stuff after).
But yeah, while Star Trek was good (hell TOS is my favorite Star Trek series to this day); and yes, it did occasionally ground itself in 'hard' science. The examples I give above also often rose to the same level as Star Trek in terms of more mature sophisticated (for TV) science fiction as well. TOS owes A LOT to The Twilight Zone as many of the same writers had crafted stores for BOTH shows.