Hey thanks for all the comments.

For now I just want to clarify that I'm pretty picky on chain tension - I always keep it limit before it derail.
When I first installed the new chain, I kept the tension very low, as usual. After only 4 runs, the chain began to develop a huge slack where it can derail easily. From there, I've decided to add a little tension (thinking the chain was breaking in) and this is where I am, with a slack-tight chain.

I will take some time today to check that out closely. I'll look if the ring is centered and try the Sheldon method. I didn't think of doing that because the S75 crank I'm using have long knurled bolts so I can't really move the chainring even when it's unbolted.

I'll keep you updated!
Best