I would max him out ASAP. I would say to him: "Joel, you are our best player. We will go as far as you take us. We will invest the maximum of our available financial resources in you. In return, we would like some sort of an insurance as we try to build this team to a long term eastern conf. contender. That insurance we seek is a team option on the 4th year of your max contract. We didn't offer you a 2+1 max deal, but a 3+1 to give you a financial lifetime security. If all goes as expected, and as we both hope, by that time you will be an All-star and MVP candidate with a team built to support you."
Personally, I think there is a bigger chance that Embiid is a healthy guy within next 5 years than from age 28+, so that is the window I'd attack in.
IMO he will be a top 5 Eastern Conference player next year. I believe in his health.