Hiding links via AJAX on pages disallowed for indexation for SEO. Good or Bad & Why?
I can honestly see zero benefit in doing this, so therefore, my answer to your question is Bad.
I have pages that I've disallowed in robots.txt and if I don't want those pages to leak link juice to external and internal links that appear on those pages.
Any properly built search engine will have the decency to obey robots.txt and not process the pages that robots.txt specifically blocks.
This means that whatever anchor target URLs are applied to that page would not have any meaning in terms of link juice distribution unless the target URL is accessible from another URL that the robot is allowed to access.
For this reason, you're wasting time trying to invent something that doesn't need inventing unless you have a crawler that disrespects robots.txt that is a good robot that indexes everything on your site which I doubt.
Why should/shouldn't I use that method?
If I "hide" these links via AJAX what SEO related pros and cons would this method give me?
I see all cons in this.
Think about the user when making your pages. Showing links only to have them vanish moments later will present a terrible experience unless you're making some game where users must guess something within a certain time limit, and even if you are, the users must be aware of it and be able to access an instruction manual so they can understand what is going on.
The second issue is that with AJAX, you're making an extra request to the server. This means it takes longer for the entire page to load, and if others access the same page, then the TTFB (Time to First Byte) will be on average higher which increases the chances of the page to be ranked lower in search results. Google page-speed insights complains about pages with a TTFB of over 200ms. This particular point may sound contradictory based on what you are doing but you asked the disadvantage of your idea in terms of SEO.