Currently I've got a strong base of clients via word of mouth and using my small network of local business owners. I'm now looking to expand, but often people don't really know they need a software consultancy to help until they have the specific way in which software can solve their particular problem explained.
Maybe naively this rules out Facebook marketing, email, Google ads and those channels. I appreciate the question is quite open ended, but wonder if there's anything I've missed.
Having said all that - at the end of the day my consultancy also get it's best and biggest projects via word of mouth. In the end I realised it was very stressful for a small consultancy to work with too many clients, and we we small (5 people), so eventually I decided we need to focus on going deeper with our current client base rather than looking for new clients. This was ultimately a superpower, and took us to a consistent 7 figurers per year with a tiny team and a few key clients.
I sold my agency last year and have recently started writing some insights on my blog https://devtoagency.com - you guys may be a bit past this (as my target reader is a full-stack dev wanting to start a custom software agency), but you may find something insightful - I actually found it really hard to find good information on the internet about software agencies/consultancies, which is why I want to try and capture my thoughts.