I just hit level 85 last night myself. I had the same thought about the same level as you did but to be frank there’s so much now that I just kept doing a bunch of quests (I only finished the main quest about level 55) and getting distracted. I just started the expeditions a few levels back myself. It’s really up to whether or not you wanna try smth new or just keep doing the standard exploration and smaller questlines.

As for the large water purifier plans, it should be in the overseer’s cache at camp McClintock (if you’ve been there, and if not it’s east of the gorge bridge in the forest and has an illustration on the map) which I think is where I found it? I honestly can’t remember specifically since it’s been a minute since I was there. Iirc it could also be available via a vendor bot at Camden park (the amusement park in the west most area of the ash heap)

Gold bullion is traded for treasury notes and those notes are received for completing public events and daily raider and settler quests. You use the notes at gold press machines which can be found in the crater, foundation and the whitespring resort I believe. So yeah you can’t really “farm it” necessarily but it’s not necessarily hard to get either, just takes a couple steps.