I don't understand why you think the only possible ways to make mergers harder need to be "analogous". If they put a hard cap on the size of companies allowed to merge, that would make it harder, without making it harder to break them up. It's hard to take seriously the idea that you think there's literally no possible legislation that could end up changing the relative difficulty in the way the parent comment describes when I was able to come up with an insanely trivial example without needing to think about it for more than a couple minutes.

The law could make mergers executed provisionally for up to X years, with a binding plan to "unmerge" that must be updated every Y months. The FTC already half-does this with post-merge divestiture requirements.