At first glance, I think a better understanding of why your sessions are so big and why the requirement for storing them for 9 months? This on face value seems really odd and dangerous, and perhaps you are doing something wrong there.
Do you have a requirement to port these sessions to the new solution?
Do you have any idea how big an individual session is? This is a factor on which session storage you can use as some have limits on their entry size.
Depending on these answer Redis would probably be a better solution as it's scalable.
You can use mysql for sessions. I am not sure off hand if out of the box you can use a different driver/connection on just sessions. If not that's probably easy to get around that by creating a custom session driver that extends the mysql one.
A quick possible solution would be to spin up a new dropplet and move everything over to it.
I don't know if there is a good devops site that would address your specific problem. There are plenty of devop consulting firms out there that would assist you on this matter.
