Remove key, replace it with vanila stick (or even something you can specify in config, nether star, emerald or something like this).
Make so this mod can be installed on server without need to be installed on client to join.

Replacing the key with an already existing item would not solve the problem because you still need the client to have the GUI config screen.

If you only have the mod on the server you only lose the in game config screen functionalities. Clients on the server still have the ability to change egg in spawner, retrieve it and break with or without silk touch and the limited spawns function also works.

The problem is...
Some server use official modpacks. from FTB client. You can't just add mod, because people will have problems joining server.
But, if your mod would allow clients without your mod to join anyway. And then, is player wish, he could download your mod, and expand ability to edit spawners.

You know mod, FTB Ultimine, this is vein mining mod. If client don't have this mod, he still CAN join server, but can't veinmine, unless he download this mod. I would like to see similar behavior from your mod aswell.

I see.
I've used fabric modloader for quite some time and forgot that forge don't have the same functionality. A fabric user don't need to download the mod and still connect - but the config screen feature is of course not available. I can looking into what can be done for the forge version.