There was talk about making sync available in Flow, but not sure if that's gone anywhere. But, also be aware that you do not want to Sync everytime there is a new user if you have a busy site, as when you Sync it clears the Portal/Pages cache and the the next user that tries to load a page will get a slow page load.
Where appropriate use the Authenticated User web role (or your own, but only 1 can have the tickbox for Authenticated users ticked), and your authenticated users will inherit the Table and Page permissions from that Web Role (it does not have to be directly assigned).
An alternative is rather than Open Registration that you use Invitations, you can set Web Roles against the Invitation, and these should avoid the cache issue when the invitation is redeemed by the user.