I believe the solution can be very simple.
Since you do have a OpenAI account (and apparently no access to GPT Plus?) you can still create a assistant and upload the files you want to work with into the conversation.
When Code Interpreter is enabled you can read and update the files as needed. Ultimately, you will get a download link that you can access from the Files section.
In case of Sheets I suggest you download your file as csv and then upload it to the assistant.
A few tips: select a cheap model like GPT 3.5 turbo where possible and keep an eye on the usage tab to monitor your cost. Also, don’t upload files outside of a conversation as this will quickly become costly when the model reads them every single message regardless if you need it or not.
I hope this is more helpful!