What authentication methods are a good fit for a custom 'customer portal'? when looking into the Oauth2.0 flow the refresh tokens are limited to 7 days which means giving consent all over again every 7 days. Preferably we would like to make use of the standard customer accounts + customer center roles so we can create and manage customer accounts from the 'access' tab in Netsuite.

develop the portal using the standard SPA web framework of your choice (or no framework) and host it in the file cabinet. This makes it possible to just use the existing NS login.

This is nice info. thanks!

You can then use a restlet/suitelet to be the data backend, all authenticated.

yes thanks for this hint. seems to be a bit simpler. Do you happen to know of any reliable way to customize the login screen as well then?

not really. Maybe via SCA, though I haven't worked with that combination (SCA login with a line of business SPA, which these often are)