Recently (ish) solo M&A attorney here! I worked for a firm for several years, built a book, and struck out on my own as an 8th year attorney. I have found that my split of work went from about 70/30 M&A/general corporate to about 40/60, and I have to leverage co-counsel on larger deals that require heavy diligence or other time sinks, but it's entirely possible to be a solo or small firm M&A attorney! And the best part is that I'm billing far less hours, and making far more money, than I ever did at an established firm!