What exactly is your goal?
You say you want to make a One-Shot with your friends and turn that into a campaign and it looks like it is developing fine. But then you sound disappointed that your players didn't engage further with your second NPC and that with the first NPC they seemed interested, but:
continued the mission
You should not be surprised by that. A roleplaying campaign is basically just a serious of adventures, missions, quests or whatever you want to call them, which in turn are further seperated into encounters, plus some downtime between those missions. Your players are playing their mission because that is the game and it seems like they are having fun with that, this means you as a DM are doing a great job. From my experience good players will actively try to stay on course with the adventure out of politeness towards the DM, because they know this is want you put the most time in and probably want them to do.
So in the end you have a functioning game, isn't that what you wanted?
How to use NPCs
Of course NPCs are an essential part of any role playing game, but as stated before this game is about going on adventures not talking to NPCs all day, so it should come as no surprise, that your players would rather do the former than the latter. If you want your players to engage with your NPCs you have to include them in the main parts of the game. Either as part of a mission or part of the down time between missions. There are so many roles that you NPCs can fill in those scenarios:
- Make them the quest giver
- Make them an obstacle such as a bad guy or just a town guard that doesn't let them through at first
- Make it so they have to take the NPC with them on the quest
- Make it so the NPC is in a prison cell inside the dungeon and needs to be rescued
- Make the NPC the local store owner, tavern keep, town crier, city ruler or some other role in town, that they are likely to run into
Presenting game elements
As a DM your goal is to present the players with interesting things, NPCs, adventures, towns etc. that they can interact with. First of, keep in mind that your players only have limited time to interact with any element of your world during a given session. If they are having fun talking to NPC A, they might not even make it to NPC B during the session and that's okay, as long as they are enjoying themselves.
On the other hand it is perfectly normal if your players are not interested in a certain game element, such as an NPC. A tip here is to not overprepare, but only create a basic outline, so that if your players aren't interested you are less frustrated, because you didn't put to much time in it. This often happens when designing quests. Problem: "I made this really cool dungeon, but my players will never see it, because they didn't like the quest hook.". Solution: "Only make the whole dungeon after the players accept the quest.". You can apply the same principle to your NPCs.
Rome wasn't built in a day
You are a new DM, making a custom campaign world, with a custom game system, trying to introduce new players into the game while also wanting to make memorable NPCs. Doesn't this sound like a bit too much? Take your game one step at a time. First develop your gaming group with a couple of friends with some simple adventures and then try to improve on those by trying to learn new gm techniques.
Don't focus to much on making 'memorable' NPCs
Matthew Colville a famous DM with his own Youtube channel full of advice, once asked his community what they wanted to learn about NPCs and the most common answer was that they wanted to learn how to make NPCs 'memorable'. In the end this is just a fancy buzzword, that can mean different things to different people.
For many it means 'make my NPCs just like the ones on [insert famous D&D show here]'. I don't know if this is the case for you, but just as general advice: don't focus on that too much, this is not a requirement to be a good DM or have a good game. This is something online DMs do because it is a good way to engage a larger audience into the games narrative. You don't have an audience, you have players and again those want to play the game by going on adventures and not admire your NPCs all day.