If you have a freelance practice with a circulatory valuation north or upto 300k then it's time to turn into an Agency yourself. Don't join the Agency unless your gut says you should. You should realize that you've done really well so far, something is working. Identify what that something is and nurture it. It is the secret to your continued success.
As always, however, the decision is upto none but you.
All the very best.