Structuring seller's continued equity stake + involvement

I'm considering a digital agency / tech consultancy. For this particular deal, it could make a lot of sense for the founder to retain a minority equity stake in the business and to work out a very long transition where he continues doing a decent amount of work for the firm. One reason: the founder loves the content of the projects, but hates the managing and scaling topics, hence he's selling. Second reason: it's a small consultancy so there are probably a lot of relationships that are tied to him, regardless of how much I try to derisk this topic.

Any advice on how to structure this? I'm guessing a minority equity stake would be in the 10-30% range to make it meaningful and interesting to him while also still be workable for me. If he does have such a stake, should he stay on as a W2 employee? Or could it be on an hourly basis that I could basically charge to the client, since this is a consultancy at the end of the day? Something else?