Stop buying the plugin

WordPress runs a very large share of the web, and most people using it solve every problem by buying a plugin. Building them is a different and considerably better paid position.

What it changes for an agency or a freelancer. You stop paying for a licence per client site. You build exactly what the client asked for rather than something close. You can fix it when it breaks. And you can sell the same plugin to other clients, which turns work you were paid for once into something that keeps earning.

The market. Custom plugins for clients, which is steady work. Products sold on marketplaces or directly. And maintenance of existing plugins, which is less exciting and is where a great deal of the money is.

What you need. PHP, comfortably. Enough JavaScript for the admin side. An understanding of how WordPress itself works, which is the part that takes longest. And the discipline to write code that does not break somebody's site, because a plugin that breaks a site breaks a business.

The responsibility is real. Your plugin runs on sites you do not control, alongside plugins you have never seen, on hosting of unknown quality, updated by people who do not read changelogs. Writing defensively is not optional here in the way it might be in an application you control entirely.

Start by building something small for yourself rather than for a client, because the first plugin teaches you the platform and you do not want to learn it on somebody's live site.