I think the real question is whether those people are playing Minecraft at all after 2 weeks. Some people only play a little bit then put the game down until the next update. If thats the case then there's not really a point in starting a server. If they are still playing Minecraft then ask them what they're doing that makes it more compelling. I sympathize with your struggle, my friend group went through the same cycle for about a year, server 3 was about 6 months ago. Sometimes people just aren't into Minecraft as much as you are.

You should check out Xisuma's YT, he did a great 3-part series on how he runs Hermitcraft (popular Minecraft SMP)

In this he mentioned for example letting the community come up with minigames and events, that they can enjoy with the rest of the community. It's also definitely important to talk with the members about what could be done better.

I do recommend modding though, it helpes a lot with Minecraft's lack of content.

I mostly try to have a lot of projects running at the same time so I can switch around if I'm getting bored with the current project. Also try some mundane things like beacon mining, those really get some creative juices flowing!

The trick to running any server is to either do what the players want or do a thing and recruit players that want to do that thing.

If your server is dying, you need to find out WHY? Ask the players and decide if you can fix their problem as the host. Sometimes it's best to switch gears, sometimes you need to find different/more players.

In general to keep players you need to keep them engaged.

When I play with actual friends, I play how they like so they enjoy the time and are likely to play again. Sometimes it's persistent creative with strict themes, other times we might run a bunch of pvp mini games.

As a host you can always be more involved. Having a set event day works well if you can commit and plan. I would host fishing tournaments every Sunday morning. They were dead some weeks but other times everyone would show up. But the important thing is that everyone knew of they showed up on Sunday morning they could fish and win prizes. Doesn't matter what you do (scavenger hunts, pvp, racing), but advertise it and do it on a schedule.

I’m just curious, why don’t you want to run a modded server?

I usually add a couple plugins to make it more fun or give them something to do. Like recently I added mobs to eggs

Yeah, it's hard to get people to care about stuff sometimes. Sometimes you just have to let it die.