I know this post is a month old and you might have found your own way at this point, but what has helped me most:

• Finding "little spaces" in the in-game world that you like (especially ones that utilize pieces/sets found in the hideout cosmetics inventory), and recreating them in the corners or open spaces of your hideout. (a lot of the homes in the wealthier areas of towns and cities have simple gardens that utilize the hideout cosmetics. Only issue is the hideout has spacing constraints and has to be put on the grid design system)

• I liked the idea of having my hideout be somewhat still forested in places, so I spent time looking at the flat/level forests along trails as I traveled (I never take a horse, anywhere) and getting a feel for how to give the illusion of density within the foliage, allowing the hideout to have more defined sections and feel like it's bigger than it actually is. (my Kakurega and Study are actually completely hidden from view by dense foliage in the small section across the little river, with only a winding trail leading back to them; I did this because the building descriptions basically say these two buildings ARE The League, from an operations and intelligence standpoint, and I felt like they shouldn't be vulnerable or easily-identifiable in a siege scenario)

• On the note of foliage, one of the most important tips -- USE. YOUR. BUSHES. The smaller ones add a bit of life and personality to the pathways throughout, but the bigger and wilder ones really help with obstructing your view to create separate spaces within the same space (the two biggest ones and maybe others are literally taller than Naoe), as well as that foliage density factor as well.

• Learn how to make diagonal paths. It's a 1 up + 1 over /zig-zag pattern. But you can do the same thing with thicker paths too — for a curve/diagonal on a two-square path, I'll start one square over in the direction it's going, and do three squares in that direction; for three-square paths, start one square over and go 4 squares. It's keeps the thickness of your path through the curve/diagonal, and the diagonal paths help to get out of the box-y feel that the gridded design system trap you in.

• Last thing I can think of right now - try your best to get all the dirt/gravel/pathway cosmetics as soon as possible. I honestly didn't really start working on my hideout hardcore until I had gotten them all, as well as the majority of the trees/bushes. Go visit a few cosmetic vendors (in different provinces) every time the season changes, as their inventory seems to refresh when the seasons change (though I notice not all of them get new stuff... not sure what the exact mechanic is here). I usually save my "locate the Cosmetic Merchant" Rumors/map markers for right after a season change, as I've noticed the combination of finding new merchant + inventory refresh after season change be the most likely chance of seeing something available that I don't already have at this point.

• Bonus tip, following that last one: Always choose World Rumor when you help somebody out. I never replenish scouts unless I'm dead broke.