Hey thanks for leaving feedback. Whenever you reach the "relationship can't get stronger...." it just means that type of interaction won't strengthen the relationship further. Most of the characters don't have fully fleshed-out interactions just yet. The indication of your current relationship is below NPC portrait panel.
If you would like to learn more about the development roadmap, you can find it on my Patreon, it's free to view. Specific updates will focus on developing NPCs. If you would like to cast your vote on which NPC should get the priority, you can join the Patreon. And same goes with locations development.