Fair enough. I read all of the numbered TOS novels from 1 to 97 and was disappointed when the series failed to reach 100! The novels are formulaic because they adhere to a formula. The same plot devices. The same themes, even when they flesh out a character's back story.
I was reading all the Trek novels from all the series most of that time - I quit getting them all when TOS was in the mid-70s. I agree, many of those were pretty formulaic - that was one of the main reasons why I quit reading them. However, the novels you mentioned - Diane Duane's Rihannsu books, and Margaret Wander Bonnano's books from that period most definitely were not. In some ways, I really miss the books from the 80s and early 90s that gave an individual author's own "take" on Star Trek, which was often a fascinating variation on what we saw on the screen.
Part of the issue as I see it is that by its very nature tie-in fiction is constrained by the limits of the original material. While I don't read tie-ins to any other franchises, my understanding is that many of the Trek lines have been given an unusual amount of freedom to develop new storylines and make major changes in the lives and situations of the characters. This applies more to Enterprise and the 24th century series, since we know so much about the events and histories of the TOS characters and periods.
I read little TOS because I generally find those books to be the least interesting, largely because of those limits. However, I know that many of the recent books have been widely appreciated by readers here. I would recommend Christopher Bennett's recent
The Face of the Unknown if you want a good example of a TOS novel that is not formulaic, but deals with something we haven't seen before which is nonetheless consistent with what we know of the characters and time period.
Given the time period that you've identified as when you've consistently read the novels and the authors you named, it sounds like you've not read much that's been published in the last 15 years or so. Although you say you're most interested in TOS, you might want to look at some of the other series as well. The Vanguard books telling a story set during the TOS series time period that gives a fascinating look at what was going on elsewhere in the Federation with great original characters as well as occasional cameos from the original Enterprise crew and some familiar guest stars. David Mack's Destiny trilogy is pretty much the definition of an epic Star Trek story which has lasting reverberations in the subsequent novels across all the 24th century series. Christopher Bennett has done an astonishing job of fleshing out the Department of Temporal Investigations, which was pretty much just a throw-away set of scenes from a single episode. He also has a line of novels which are continuing the story of Enterprise and I find to be great reading, even though I found almost nothing to like about the TV series. Una McCormack has done amazing work with fleshing out the Cardassians in many of her novels. While they're not always my favorites, Greg Cox and Dayton Ward consistently come up with intriguing new plots and situations, often in the TOS setting.
Also, since it looks like you may be new to these boards, you might want to be aware that several of the current authors visit and post here. While I think they're probably pretty thick-skinned by now, I'm sure they'd appreciate it if any criticisms you made had some substance to them, rather than just a broad-based dismissal of them as "minor league fiction writers," especially since it's not clear that you've even read anything by the most recent group of authors.