It's basically a paywall. If you want more, you need to login,
That's fine, I'm not judging on what is the right or wrong approach, it's your business.
Let's leave this. It's an offtopic distraction.
This is where I disagree. This is not off topic, it is really the root of the problem. But clearly you feel that "sculpting link juice" will address what ever ranking issue you face. Practically speaking internal links all point to the same place, there are menus, footer links and links from other pages on the site all cross linking one to another, this funnels link-juice throughout the site. It seems really unlikely that removing an internal link would have any measurable impact on "link juice". But removing links (or adding links to unexpected locations) will certainly have an impact on usability and that can have direct impact on rankings.
Then... a form submit would prevent this, as Google stop following at this point.
I think we have a fundamentally different views of how Google "follows". Yes, with a form is not a link so it will be "no-follow". But if the user is shown a form and drops off, and returns back to Google, then Google sees that and uses that information. It sends the signal that the page does not meet the user's intent.
Basically what I'm saying is whether or not the links is a "form" or a "href" will have little impact on ranking. What will make a difference is that when the user clicks they get they expected result.
If it were up to me, I would make it such that when the user clicks the "link", if the user isn't logged in, they are shown a login modal(popup) that explains the paywall, and provides the user some offer, and if the user doesn't want to sign-up/login then let them close the modal and continue to view the page they were on, and if the user signs in then they are redirected to the destination page. In either case the user gets to continue their journey with minimal friction.