I landed on Claws Mail myself. It does look a bit dated, but the UI is functional and the client works. I’m content with it.

Anecdotally, I don’t trust Klarna one bit. A few years ago I was making a payment and Klarna directed me to a third party site that wanted my bank credentials. It was at that moment I stopped using Klarna and requested they delete all information they have on me. Apparently, Klarna wanted to see my account activity. I now go out of my way to avoid them. I have a similar issue with PayPal, where connecting a Credit Card requires entering information in a way I’m uncomfortable with.

So for now I’m just using a CC directly. This may not be the best approach, but it feels the least intrusive - for now. I don’t know what the optimal solution would be. Maybe when GNU Taler becomes an option…