I am a marketing consultant based in San Francisco with more inbound requests than I can handle. I am well aware that I could make this an agency, but I am not sure that my margins would support the work. How have other solopreneurs scaled their consulting practices?
This ultimately comes down to two things:
1) What's your vision / end game for this business
2) If you do scale your business... will it become more valuable or less valuable?
IMHO, you don't want to start an agency unless you want to enjoy the fruits of a negative cash cycle, dealing with pissed off clients on a daily basis and the very real probability of a difficult earn out period should you ever wish to sell the business.
So, do you want to sell it one day?
The fact that you're contemplating starting an agency means that you're probably offering a broad scope of services, and as a solo outfit... it's impossible to deliver 'everything' unless you consider outsourcing the work or starting an agency... hence your current predicament.
You can scale as an agency, but ultimately it doesn't mean you will make more money or provide a better service for your clients, and if you want a business you can sell in the future you probably don't want to start outsourcing or start an agency.
Sure, you can start an agency, but then you need to specialise in something if you want to scale it effectively and perhaps one day sell it.
The hardest part about being an entrepreneur / business owner...is to say the word NO.
Scale doesn't always = more profit.
This all comes down to your vision.
How you scale is by deciding on the service that you offer which creates the most value for your clients and which makes you the most money / profit.
Once you're there, develop SOP's which you can hand over to a sales team and or a team of resources that can deliver the work on your behalf.
You then spend the rest of your time selling your company and not the service itself.
You'll be happier for sure - you'll make more money (by scaling effectively) and if you do decide to sell you can... and without the hassle of earn out periods.
But again, this comes down to your vision :)
Good luck :)