I found a workaround for the "only 1 mod" thing.
At first, I couldn't use your mod with the "Constants" mod. I suppose it's beacuse they modify the same file? It shouldn't be an issue according to the modding guide, but apparently it is.
Anyhow, to "fix" this problem you can just copy the code from the mod files (in this case "mainmenu.gd" and paste it in another mod's file (as long as the files have the same name). I have no idea why the problem occurs or why this works, but I currently have "Constants' active on my new story-mode Dragonkin.
Keep in mind though, in the in-game mod menu, only activate the mod that has all of the codes.
In my case, I took the code from this mod and pasted it into the "mainmenu.gd" file of the "Constants" mod, then I just activated "Constants" mod. If I activate the "Any Race" mod along side the constants, they both fail.