I have a similar set up (with a 32gb built in SSD). I use it when traveling. In my experience running an OS off of an SD kills the card in about a year and it dies without warning. Make sure you don’t save anything important on there without a back up.
Love what you’ve done here and can’t wait to see more of your projects.
Just booted mine up and you’re correct about the size of Mint Mate. It’s about 14Gb. I don’t remeber what OS was 4Gb.
Also for extra security I added 2FA to my sign in. Anytime I enter a login password I need to get a code from the authenticator app on my phone.