In the TOS episode 'The Ultimate Computer', we see the problems with the M5 unit by Dr Daystrom. I know in TNG, and even VOY, there is still need for a crew aboard the ship, but is there any further reference to what becomes of the M5? Like, is an M6 ever developed or something or is this sort of thing ditched and all computer developments focused on improving human-computer interfaces, as seen with the LCARS system in TNG onwards?
Within the Trek TV canon there are no further mentions of the M5 computer beyond the episode "The Ultimate Computer".
Within the various EU books it's mentioned pretty regularly;
In Star Trek - New Frontier : Gods Above - As a (literal) textbook example of why engram-based computers are a bad thing.
In Star Trek : Next Generation - A Rock and a Hard Place - As justification for not allowing Mr Data to take control of the ship.
In "Strange New Worlds 6" there is an entire story called 'The Soft Room' which features M5.1, the predecessor of the M5 computer. The main plot revolves around the M5.1 taking control of a Romulan research facility and trying to find a method of escape.
There is no mention of a successor to the M5 ever being developed, though I do recall a novel (which I can't find a reference to without remembering the name, unfortunately) where Daystrom's work is continued by a certain Dr Soong. In a non-canon novel, The Rift, Richard Daystrom invents the Holodeck, but otherwise he personally seems to have faded into obsurity after the M5 incident.
It appears that duotronics and multitronics were almost entirely abandoned in favour of isolinear circuitry, however, according to the Memory Alpha article, multitronics are still in use to a limited extent, forming part of The Doctor's holomatrix (some kind of software-defined variant, I would presume) and are also used for a type of neural interface.