Thanks for confirming model by serial prefix. Yup - that's an 8GB unit.
Although you say "..it's not as if I'm adding any maps...", please appreciate that each release creates bigger and bigger maps as more detail is added (more roads, mapped parking lots, more known speed limits, etc etc). I would have thought you'd have been OK with an 8GB unit, but we don't know what else is being stored on it, and the unit will also require a minimum amount of unused space to load and pre-check any new firmware before it replaces that during a firmware update.
You could go with another 8GB on a Class 10 uSD card in the available slot. Your unit will see the new card, ask to format it, and then when you next connect to MyDrive, you will be given some different options when you go to update again.
As for the SSL error, are you describing a certificate error on the part of TomTom's server? That shouldn't be happening on their production server. Details, please, if you have them.