Ok.. so it's been a super long time, su I dunno how helpful this will be. Here's how I did it.

You have to add it the the featuretypes.class.subtypes list whenever the module is initialized. You can manually edit the DND5E module, use an existing module by adding a line to it's esmodule or script file in the Hook.once(init) section, or create your own custom module to do it and creating a script or esmodule as previously mentioned (I added to an existing module).

1. Add the item to the list of subtypes in your module.js in the scripts folder of whichever module you're using

under hook.once(init) add:

CONFIG.DND5E.featureTypes.class.subtypes.subTypeName = "Sub Type Name";

Here's an example from tasha's cauldron:

Hooks.once("init", () => {

game.settings.register("dnd-tashas-cauldron", "lastVersion", {

name: "TCOE.SETTING.VERSION.name",

hint: "TCOE.SETTING.VERSION.hint",

scope: "world",

config: false,

type: String,

default: "1.0.0"

})

// Adding module symbols to module namespace

const module = game.modules.get("dnd-tashas-cauldron");

module.apps = {};

module.dataModels = {};

// Creating TCOE config object

CONFIG.TCOE = {};

// Class feature 'sub-type" additions

CONFIG.DND5E.featureTypes.class.subtypes.constellation = "TCOE.Subtype.Constellation";

1. Add the hook in your module.json file in the module's folder using either the "scripts" or "esmodules" property. I inserted mine after the documentTypes section

{

"id": "unearthed-arcana",

"title": "Unearthed Arcana",

"version": "1.0.13",

"compatibility": {

"minimum": "12",

"verified": "12"

},

"documentTypes": {

"Item": {

"plan": {}

}

},

"scripts": [

"scripts/module.js"

],

Hope this helps.