I have ongoing email discussions with the support about this.
Currently it looks like that all fonts are loaded that are in the style guide.
So, if you change everything to Arial, which is an internal font, Google fonts are loaded if they are in the style guide.
Looks like in the attachment und er site styles / fonts
Currently, there is no option to remove fonts from the style guide.
Found this solution https://e2a.studio/squarespace-tips-how-tos/how-to-remove-unwanted-fonts-from-your-squarespace-website but it's a lot of work and didn't work for my template.
Still waiting for a solution from Squarespace to be able to use Sqauarespace legally in Germany....
I'll kepp you posted!