There's probably a word I've been pronouncing wrong my whole life that I don't know about

https://discuss.tchncs.de/post/10066578

There's probably a word I've been pronouncing wrong my whole life that I don't know about - tchncs

A "niche" is not a "nitch"

You're a bit too late for trying to complain about that one.

The latter has been the dominant American pronunciation of the word for so long that it now appears as the primary pronunciation guide in American dictionaries.

Definition of NICHE

Definition of 'niche' by Merriam-Webster

I hope cache isn’t pronounced like cachet (rhymes with sashay) rather than cash.

Australians pronounce is as kaysh which Ive always used, and I was horribly annoyed by Americans pronouncing it cash.

I was even more annoyed when I learned that cash is the “correct” way to pronounce it!

Get used to it, my Australian friend
It varies by region at least in the US based on a few years of doing service desk work. Listening to YouTubers, it seems a bit all over the place as well.
It likely correlates with French influence in the Sputh, seeing as it is a French word.
Some of the the Louisana folks would often say ka-SHAY in a wonderful Cajun accent.
They’re exactly the ones who should know better…
Well, crap. Guess I'll eat a nice quitch to chear myself up
You mean a quickie?
As long as it's not scrambled, I suppose

Both Oxford English Dictionary and Merriam-Webster agree that “nitch” was the correct pronunciation in both British and American English until very recently. You already linked Merriam-Webster, so here’s O.E.D:

N.E.D. (1907) gives only the pronunciation (nitʃ) /nɪtʃ/ and the pronunciation /niːʃ/ is apparently not recorded before this date. H. Michaelis & D. Jones Phonetic Dict. Eng. Lang. (1913), and all editions of D. Jones Eng. Pronouncing Dict. up to and including the fourteenth edition (1977) give /nɪtʃ/ as the typical pronunciation and /niːʃ/ as an alternative pronunciation. The fifteenth edition (1991) gives /niːʃ/ in British English and /nɪtʃ/ in U.S. English.

(N.E.D is the original name of the O.E.D. “/nɪtʃ/” is pronounced “nitch” and /niːʃ/ is pronounced “neesh”.)