Okay, I think I figured it out. Try doing this:

1) Place down all of your respawn markers in the editor, and call them whatever you like (they don't have to named anything in particular in this case in order to function as respawn points).

2) In your description.ext, make sure you have the lines:

respawnTemplates[] = {"MenuPosition"}; respawnOnStart = 0;

You can add additional things to the respawn templates (https://community.bistudio.com/wiki/Arma_3_Respawn#Respawn_Templates), but make sure to at least have "MenuPosition" if you want them to be able to select their subsequent respawn points after their initial spawning.

3) In the same mission folder as description.ext make a file called init.sqf and put lines like this in for each marker you want them to be able to respawn at later (I made three, which I named "r1", "r2", and "r3"; note that in the Eden editor that's what in the Variable Name field for the marker, what you put in Text is what the position will be called in the list of respawn positions if you have "MenuPosition" in your list of respawnTemplates as shown above). So my init.sqf would be:

[west, "r2"] call BIS_fnc_addRespawnPosition; [west, "r3"] call BIS_fnc_addRespawnPosition; [west, "r1"] call BIS_fnc_addRespawnPosition;

(Note that "west" means that only BLUFOR players will have these spawn points available to them, but you could also use "east", "independent", "civilian", or "missionNamespace" for everyone to use).

The important thing here is that the players will initially spawn at the last location in the list. So in this case they'd all spawn at marker "r1" by default. If respawnOnStart = 1, they would have a map allowing them to choose their position at the start of the mission, but setting that equal to 0 disables that spawn location map initially, but means that after they die the first time they'll have it available for all subsequent spawns. Give this a try and let me know if it does what you want it to do.