I’d say your assumption that the writing is generally poor on tie-fiction is unfair and unfounded. If you like reading science fiction, particularly space opera, you’re missing out on some really great books.
Yes, there are some books that are clunkers, and some to-be-avoided tie-in writers, but at its best Star Trek novels are just straight up excellent science fiction.
I read extensively in the genre and I can’t say unequivocally that the quality in non-franchise science fiction is generally higher. There are regrettably a high number of unreadable but books in each crop of new and recommended Sci-fi offerings.
What I can say is that the ‘put all the toys back where you found them’ books written while shows are running are constrained by that. While it’s a genuine problem for some, books by DC Fontana, Diane Duane and Vonda McIntyre are all worth your time.
The post-Nemesis Relaunch novelverse provided a complex set of tales that were allowed to take 24th century characters and societies forward. It’s been overwritten by Picard, but there’s still a great deal of high quality content for those of us who are voracious readers. There are also some 23rd century books, including the Vanguard series, that have continued.