You can create a group policy that bypasses the splash page and then apply that group policy to any user you want to be connected for more than 3 months.
If you want to automate the solution, you have a couple of options. You can host your own splash page and make an API call into Meraki to apply the group policy after the user clicks through your splash page. You can also use MAC-based access control with a RADIUS server and set the group policy in the RADIUS reply.
You might also want to open a case with Meraki to see if they have any other configuration options available to you.