You have two options:
1. Cultivate more valuable expertise
2. Increase operational efficiency
Those two options are, in some cases at odds with each other, but if you focus on #1 first you can use that expertise to drive #2 later. Or, you can ignore #1 and focus exclusively on #2.
#2 looks like productization. Ex: https://knapsackcreative.com/ You basically trade the occasional dramatic upside of custom work for the more consistent upside of standardized work, which lets you iterate your way towards a highly efficient operation and gain incremental improvements in profitability along the way.
#1 requires that you specialize. This does two things. It addresses marketing efficiencies by helping you answer the questions "who are my best clients?", "how do I find and connect with the?", and "what job are they wanting a website to do and how can I help them do that job better?". Second, it allows you to cultivate exceptionally valuable expertise.
About expertise: Skill gets it done; expertise ensures it has meaningful impact for your client.
If the most you've been able to earn from building a website for a client is $2,000, you probably have skill but lack expertise.
Specializing in a market vertical (ex: manufacturing, retail, etc.) or a type of website (ex: e-commerce, membership sites) will accelerate your ability to cultivate expertise. Repeated exposure to similar-enough situations will allow you to move beyond offering just skill to offering impact, for which clients will happily pay higher fees.