Have been considering this a lot with respect to our trading platform never wanting to see a customer's keys. The answer seems to be to simply allow users to send their data to the API along with generating an encryption key on server and having additional services only forwarding secure data in a relay type manner. Never saving tokens in the browser at all.
An alternative could be a desktop app. We have around 90% of the server infrastructure in place. Pretty cool stuff with Nostr keys for the first iteration.