Creative deal structures for a marketing agency?

Looking at a niche marketing agency for sale with gross revenues around $4M and SDE of $650k for a single owner. High client and industry concentration (top 5 clients make up 72% of revenue) and owners have decades of industry experience. Despite this, I see potential growth opportunities with this company and I have a background in this space (on a smaller scale). However, I recognize the financial risks and trying to come up with a few creative ways to structure a deal besides the typical equity/debt/seller note model. I think seller's valuation expectations may be 2-3X SDE. They'd open to an earn out of some sort. With the client concentration being high, I think that's going to be a red flag for some (or many banks).

Are there any structures to buy in over time during a transition period, similar to a lease to own with real estate, or some way to give the seller greater valuation out of future revenue and/or the lift on growth? Just wondering if anyone has gotten creative with their deal terms and thought of something I haven't seen yet. Thanks in advance.