Yup, it's only and just for spending online. Since it is shared between two parties, it has to depend on the security strength of the other party, but how you trust the other party is another matter.
By the way, how much can you deposit into a mnemonic with this kind of mechanism? I would deposit about $10,000...?🤔
Would depend on the client and the exact people I'm sharing with, if it's just me and it's not going anywhere on my local app that's open source and verifiable, $10k makes sense (since you wouldn't want more than that on device anyway). If it's on relays, there could be a Save Now Decrypt Later attack so not more than $1k , plus it also opens you up to metadata attacks if the events are not relayed properly.
