What you describe is more of a design flaw than a scaling issue. Thinking ahead that custom fields could be needed in the future is a design choice and can app

What you describe is more of a design flaw than a scaling issue. Thinking ahead that custom fields could be needed in the future is a design choice and can apply to 5k users or 500 000k users. It's not the scaling itself that causing you pain, it just exacerbate the difficulty of a bad design choice. So advice of OP still apply, at start you should take most of your time develloping the best design for your application and less on scaling. Because scaling a good design is way less painful.