Signpost uncommon for the (White-black archetype), which as discussed on Expansionist Princess, is a mana-hungry mechanic that appreciates you making your land drops. Somewhat fighting for conceptual space with the Expansionist Princess... Not sure if they can both coexist in the set or not.

Digging for lands really feels wrong in WB. White could get plains and black could get swamps, but that's really it

Without playing it (obviously), decadence looks like a tough mechanic. You're rewarded for having more of it, but there's a tension in having too much. Very spikey and somewhat antilinear

You're not wrong. I have white fetching Plains on Expansionist Princess and I've been wondering about a black-mana-doubler like Crypt Ghast, but I could also have a Swamp-fetcher. Originally this card's design did just call out "Plains or Swamp cards" but that felt even stranger to me.

It is a problem with this design. Another problem with this design is the huuuuge amount of woooorrrds. So it might need killing and rethinking.

As for decadence overall - you're right that it's got a bit of a tension in having too much. I think it'll generally be quite hard to get so much that you literally can't pay it, so I think in a heavy decadence deck you just get cycles of a turn or two when you pay your decadence and get all the effects but don't get anything else, followed by a turn or two where you're not paying decadence, just playing creatures and other cards. That seems fine.

It is somewhat antilinear whilst also being somewhat linear, especially the way that most creatures with it only have decadence 1 so you want a bit more to really get the effects going.

As for how it plays? It's been some time since the decadence playtest I did so I don't remember it too clearly, but the notes here and here sound like it played fairly well.

Maybe in fact the thing to do is to just admit that black-white decks won't be able to play too many decadence cards (unless they get the Expansionist Princess). That's OK because the (Green-white archetype) is all about using various green ramp cards to help pay for decadence. Does mean I need to figure out what the white-black uncommon and archetype should be though.