Personally, I’ve gotten a lot of mileage and income from knowing MSSQL databases, ASP, C#. On the frontend side – jQuery, Bootstrap, JS and some Angular, and VueJS frameworks.
Learning WP, theming, making custom themes won’t hurt. It’s easy and you can land jobs that may be anywhere from few hundreds to a a few thousands that are no-brainers. Very easy, low-stress jobs. (of course, it goes without saying these are not jobs you’ll find at freelancer or upwork. Those guys are crazy, racing against each other to the bottom.)
Knowing other related skills to web development or graphics can bring additional income – photography, video editing, designing CD packaging, motion graphics, etc.
Having other sources of income – selling stuff (hardware, software, services), affiliates, commissions, adwords, etc. also helps.