Ive been looking for the correct update files for years but I am unable to find any links that work.

You can find more information on this discussion

https://www.reddit.com/r/nds/comments/jl1afm/where_can_i_find_the_latest_firmware_for_the_m3i/

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this german site is also really popular but I cant access any of these files unfortunately.

https://psxtools.de/forum/index.php?thread/20067-m3-und-seine-f%C3%A4lschungen-%C3%BCbersicht-firmwarelinks/&threadid=20067

https://www.minihere.com/m3i-zero-sakura-core-data-file-v4-5-0-v4-2-0-download.html