Learn your way from the bottom with small clients, then progress to larger clients. I thought I knew everything after working for years with dozens of clients spending $10k per month. Only in 2018 did I work with clients spending $100k per month and it is a whole different ball game. The job is much more demanding AND it is much more interesting!
There is a huge shift between these two budget levels, time is spent on analysis and reporting, but it is extremely valuable. Spending 10 hours creating in depth analysis goes much further than making many many small tweaks.
Grow fast, then trim the fat. Grow fast, then trim. Grow fast, then trim. This isn't possible with small clients, but it's such an amazing way to progress.