I understood you.
I didn't have any answer from Cardinal.
I did plugin for nopcommerce and Cardinal but documentation is very very bad.
I noticed that Cardinal started to do the new documentation.
I did plugin for Baintree
with last API, but Baintree doesn't support PSD2 yet (you can check it yourself. 3D Secure is beta now. And Baintree will use Cardinal API for 3D Secure 2.0 :).
When Baintree adds support 3D Secure 2.0 (SCA, PSD2) I'll add it in my plugin.
3. PayPal Hosted solution.
I have plugin for it (PayPal Pay Flow
4. PayPal Direct Payment
I don't have any solution.
PDS2 (SCA) doesn't allow to store credit card in any places.
It's only solution to use external IFRAME. Maybe it's a reason why PayPal did PayPal Smart Payment Buttons
But I can't understand why PayPal didn't do easy solution like Stripe.
We'll be wait.