So, how does one win?

[Grognerd]

I've explored roughly half the house, begun to poke around the basement, cataloged dozens of books, read quite a chunk of them, leveled skills to 4-6 and unlocked new soul cards for the same, leveled a bunch of those soul cards, took note of many crafting recipes, redecorated the stairway with busts, etc etc etc ...

But I still I have no idea of what I need to do to "win" the game. I figured by now the game would nudge me in that direction somehow ... but nothing. The only marching orders the game gives you at the outset if that you must restore a fire damaged house (via unlocking rooms, which I've been doing obviously), but as books and recipes and many other things are largely irrelevant to this, I'm guessing a full house reveal is not really a victory.

So ... don't keep me in suspense any longer! WTH am I supposed to be doing here?!?!?!

[Super_]

your journal is your goal

[Andi]

Find and unlock the "room" that contains the Earl's Yew. That "room" is called Earl Brian's Field.
Then you can decide. Are you OK with a minor victory? If yes, just use your journal with the tree.

If you are ambitious though, and want a major victory, you need more.
To be precise, you need to find a book that contains a Numen. Some of them can be found in rooms that are comparably difficult (but not really difficult) to unlock. Others are hidden a bit better, but you can find hints in various books.
In addition you'll need high level ink (Encaustum Terminale). You can either craft those, or find some of them around the house, especially in the more difficult to unlock rooms in the basement.
And last, but not least, you'll need a ton of aspect. I'd recommend to upgrade a matching skill ("matching" as in, matching the Numen and the Encaustum Terminale you want to use) to level 9. You might also need to upgrade a matching soul aspect.

Then use the Numen and your journal, and enough aspect, to make a Dedication.
Next, use the Numen again with your journal, adding the Encaustum Terminale and the level 9 skill, to write a History.

Once you have that done, you "only" need to wait for the next season of Numa. When that's here, bring the journal to the Earl's Yew.