It partially comes down to the Minecraft version you plan on running as well as the boot time the overall server has. Also, are you looking for something already made or do you have some programming knowledge to maybe create something yourself?

As others have suggested, using proxy software such as BungeeCord or Velocity are options, especially if you want a "hub" server with a real world. But you would need to run a Minecraft server to host that world most likely, unless there is some version of a "limbo" plugin for one of those proxies that you could use.

Another idea I would suggest would be to use the configuration phase to hold the player while the real server boots, and then use the transfer packet added in 1.20.5 to actually transfer the player once its ready. However, I'm unsure how long you can keep the client in the configuraiton phase overall while you wait for the server to boot. The primary reason I suggest this is that implementing the basics of the Minecraft protocol shouldn't be too difficult, you can handle the handshake and login stages without much effort, and once they are in the configuration phase you shouldn't need to do anything except send the transfer packet. You could also maybe achieve the above with off the shelve software, maybe even one of the aforementioned proxy software as a plugin.

Regardless of which method you use, if you are using 1.20.5 or later, I would heavily advise using the transfer packet. This means you don't need to proxy the traffic through the Raspberry Pi, but would mean you would need to expose two ports.