I think you’ll need a plugin.
Can you describe the problem you’re trying to solve rather than your proposed solution?
If they aren’t custom user fields, what kind of custom fields are they? They need to be connected to something. Maybe you want a custom model?
Maybe you want to check out Developing Discourse Plugins - Part 1 - Create a basic plugin. Or if you know something about programming or Rails, maybe look at GitHub - discourse/discourse-plugin-skeleton: Template for Discourse plugins and discourse/lib/plugin/instance.rb at main · discourse/discourse · GitHub