Minecraft – Why won’t the villagers breed in Vanilla Minecraft 1.15.2

Always leave server-ip blank unless you're doing something unusual and you understand what "IP binding" means and you know that you need Minecraft to do it.

That IP address you have there is your external IP address. (I checked.) However, your actual Minecraft server machine does not own that IP address because it's behind a router on a local network (LAN) and owns a completely different, "local only" IP address assigned to it by the router.

What setting server-ip to your external IP address has effectively done is told Minecraft to ignore all connections to all addresses except that one. But since your server machine doesn't actually ever see that external IP address (remember, it's real IP address is a different, local-only one provided by the router), that means it ignores all incoming connections.

So blank server-ip and you'll be able to connect.

This applies to Minecraft versions 1.8.0 - 1.13.x

The 1.14 Village and Pillage update changed breeding mechanics again

That your Villagers are entering into "mating mode" (i.e. they occasionally have hearts floating out from their bodies) is a good sign. It implies that the Villager's surroundings are suitable for breeding to occur.

However, with the 1.8 release Villager breeding has been tweaked. Villagers now also have to be "willing" to breed, being in "mating mode" isn't enough anymore.

From the Minecraft Wiki:

Willingness

Additionally, villagers must be "willing" in order to breed. After mating, they will no longer be willing.

Villagers can become willing by the player trading with them. Willingness is granted the first time a new offer is traded, or at a one-in-five chance on subsequent trades. This will not cause them to immediately seek out a mate, however.

Villagers can also become willing by having either 3 bread, 12 carrots or 12 potatoes in one stack in their inventory. Any villager with an excess of food (usually farmers) will throw food at other villagers, allowing them to pick it up and obtain enough food to become willing. You can also throw bread, carrots, or potatoes at the villagers yourself to encourage breeding.

That would suggest that it's this new "willingness" mechanic that's actually holding your village back. As the article suggests, you should be able to make the Villagers more "willing" by throwing enough food at them. A caveat though: don't expect this to be quick when your initial population is small!