It's probably tight enough for your needs. Unless you live in Switzerland or are breaking Swiss law, they'd need a really good reason to send your data anywhere.

That said, I use Tuta. They have a similar source model (open client, closed server) and are based in Germany, but since they're an underdog, they have a bit more value and lower costs. I pay €3 and get 3 custom domains and 15 aliases, whereas w/ Proton I pay $4 and get just 1 custom domain and 10 aliases; I can also add people to my plan for €3, instead of upgrading to a Duo for $15 or family for $24. If Proton matched Tuta's features, I'd probably pay slightly more for the better UX, but I use those features so I'm very hesitant to give that up. I don't intend to use their VPN or other products, so I'm very much not interested in their higher tiers.

I do wish their server code was open source and self-hostable. I'd love to use my own storage, but still use their spam filtering and whatnot.