I loathe ‘premium’ WordPress themes’ custom options pages. Not everything should always be configurable. That said, Kirki has helped me extend _WordPress’s built-in Customizer_ on a few occasions. https://wptavern.com/build-customizer-settings-faster-by-using-the-kirki-framework-in-your-project
Also, 100% open-source. https://github.com/aristath/kirki
Server run by the main developers of the project It is not focused on any particular niche interest - everyone is welcome as long as you follow our code of conduct!