I’ve ran into the same thing. I’m surprised that this setting even exists, and that it has a default > 0. This is IMHO an unpleasant default behavior, and puts burden on site admins to fix it.
I understand the desire to limit “noise” in a public discussion, but 1:1 conversations are completely different. It’s fine to have a limit on the first message to avoid low-effort “hi”, but not for replies. If someone has sent the first message they’ve already given an implicit permission to receive a reply.
I don’t think the heart button is a sufficient substitute. It doesn’t feel like a proper reply to me. It is a bit ambiguous (is it merely an acknowledgement, or did I love the message?). I want to reply with a manually typed text, even if short, because it feels more appropriate and personal than just pressing a button.