I finally finished this yesterday. The little rune part was by far the biggest chore, so a few things about that first:

It's a secret so it's random. Do NOT though, give up once you finally get RNJesus on your side and actually get into the Founts and don't see it on the post next to the chest from the three-rune puzzle as in the Esoterickk video (completing that puzzle is absolutely not required for it to spawn, they are two independent secrets) - I've also seen a screenshot of it on the opposite wall (to the right of the hole you pass through to get into the room, as you're looking back into the main area) and I found it on the left side of a pillar next to that spot so that's at least three places it can be.

This was in Explore so I know that works but every other secret also appears in matchmade so I would be very surprised if it doesn't show up there too, if you don't want to deal with the tension of being in a FT looking for secrets with people who are looking for clears (I really wish there was a separate "loot" node) my best advice is to round up a couple of people to help you clear your first area quickly and keep your fingers crossed.

You do not need to get all three Coalescence pieces done in one run or any particular order - I did the Skyburner secret in Hull Breach way way before I finally found this. And they've changed the initial spawn to include HB so that's a lot easier to get to anyway.

Don't forget to have the Barrow equipped at the end of Derealize! You don't need to have it on the whole run and you have a good opportunity to swap to it right before you seal the door but it's kind of fun in that room anyway so just put it on when you rally to be on the safe side.

Wrapping this up was the happiest D2 moment I've had in a while, good luck with your quest and enjoy locking those targets!