Honestly.
Canon I think is a personal choice.
Let me explain.
If one actually analyzes the timeline of things and especially with stuff like multiple dimensions and time travel, obviously any sense of real continuity can be easily dismissed because it all really doesn’t make sense if you put too much thought into it and I think it’s for each individual to decide.
For me, core trek as I like to think of it, that being.
TOS - TOS Movies - TNG - TNG Movies - DS9 - VOY
All form a RELATIVELY cohesive narrative as far as world building goes, yes you can break it down and scrutinize each individual line about technology and terms that were used but ultimately, there’s a semblance of a history that takes shape that is compelling, at least to me and when I view Star Trek, it simply helps with my enjoyment and immersion.
For myself, I completely discount ENT, Discovery, the Kelvin Universe, Picard and of course, Lower Decks.
If you don’t, nothing wrong with that, just the way I like to look at it and how I choose to enjoy Star Trek.
So how important is canon?
As important or not important as YOU want it to be.