Fedimint and cashu pretty much rely on lightning to be interoperable. This stack is insanely complicated

I like the idea of having something like cashu, but directly correlated to onchain BTC. If I’m understanding correctly, isn’t this what Ark is?

Yes, we still need it. Everything comes with it's own tradeoffs.

Actually ecash, Cashu/fedimint don't rely on a specific underlying funding technology. Lightning is nice because it gives instant settlement which is great for the swap ux. It could easily be interoperable with other shitcoins, L2s or even good ol' paper. It's just custodial tech