Using Google Sheets doesn't add 3rd party tracking back in. Even if you don't trust Google to not take a look at your spreadsheet, it's unlikely anybody or anything at Google would treat your spreadsheet like analytics data.
All that being said, it is a little funny seeing them funnel the data right back into Google. Personally, I would also prefer a different storage method.
It's not really a hard requirement for me to get off Google though, just to avoid using their ad-tech. For privacy concerns, using Google Sheets isn't all that different from using MySQL on Google Cloud, etc.
On that thought, you could potentially hook it up to Google Sheets or editors that integrate with remote git repos. Skipping 3rd party hosts, you could host your own git server for cheap too.
Awesome project nonetheless! I'm a hypocrite still using GA but want to move off of it this year, so reading your solution has me thinking about my own.
I'm all for not using Google Analytics- I don't use it on my site. But why would you then use Google Sheets to hold the info? That's ridiculous. That's like telling people how to make an apple pie without sugar, and the last step before baking is dumping in a pound of sugar!
If you want an easy data store, use something like SQLite. There are plenty of options that are easy to self host, and a lot of libraries have been written to make it easier for you.