PHP + Mysql and memcache or phpsessions

PHP + Mysql and memcache or phpsessions

I currently manage a game that handles around 8K daily users, the current
server setup is
VPS: 16GB+8cores+ 160GBSSD.
Handles 8K daily users that are generating 600K daily direct calls to the
mysql DB;
The situation is: The game is growing, and server response is slowing down.
So! I'm looking for options to avoiding having an incredibly slow server,
and saw Memcache as a viable solution, and studying it I came up with 2
questions, which are:
Is it safe to store ONE Key value per user, that contains all user data,
perhaps as a concatenated string that would be around 100 characters long,
and then update the DB every once in a while? ->that would mean having
thousands of keys at the same time.
The idea is not to replace mysql, just to aid it in the users session and
be able to update the DB every so often, for this situation would Sessions
or Memcache be the right way to go?
thank you very much :)