There are multiple posts I found
This description of Iroh mentions him departing to the spirit world, but it doesn't sound like anything fancy. Just him simply deciding to leave the physical world behind and being accepted into the spirit world. Considering he has a stronger connection to spirits than anyone else in the series, short of being the avatar and a partial spirit, this isn't too surprising.
This other stack exchange mentions how he had time to visit the Spirit World to search.
Anything else I found was a repetition of things you already know.
As for abilities he has gained, I would assume nothing changed. He gave up his physical body to live in the spirit world where bending doesn't work (without using the portals, which he can't use anyway anymore). Traveling to the Spirit World has yet to provide anyone with additional skills, just knowledge.
Although, if you want to stitch some things together, given the previous instance of the Winter Solstice and Jinora's spirit projection abilities, I wouldn't be surprised if he could appear in the physical world under optimal conditions.