Say I want to actually give liquidity? Any tool you can recommend for that?

You can do it on Magma and Lightning Network Plus but I personally avoid using them because they require accounts whereas their competitors don't I'd like to see an open source alternative to those where you *don't* need an account

The advantage of a p2p #nostr liquidity marketplace is that anyone can build on it.

You can use publsp to sell liquidity

I'm currently working on a simple website that runs the customer backend of publsp to allow anyone with an internet connection and a wallet to purchase liquidity from LSPs advertising over #nostr. Having an Umbrel/Start9 app for it would be fantastic for LSPs.