Tabe nice video, thanks for sharing! I didn't realize you had a channel, I subscribed
I got the shadow M on preorder from SCS. I have a labs red + hand magnetized wuhua v2. Compared to both, both the inners and outers on the stock shadow M feel weaker. Still with a noticable click and I like the strength personally, but the difference is apparent when turning the puzzles.
One common theme between all the shadow M reviews I've seen - major catching and even popping on stock tensions. Everyone seems to need to tighten the cube up a lot. Sounds like you already did more work on the puzzle but if SCS kept the stock tensions it would explain the OOB experience.
I'm optimistic, I think you can get that cube working wonderfully with more setup. I still need to dig into mine, will post notes here when I do - I'm planning to lube the core, carefully lube the inners to make them a little more cohesive, put dignitas on the outer, and tighten the puzzle up.
I'll open up the M and look at the magnets too - I don't have the gear to measure the strength but I can at least figure out the sizes in use.