Hi all, Grist has reached out to help customers and users looking to migrate from Spreadsheet.com. Here is a note I received from them:
The Grist team is happy to welcome Spreadsheet.com users looking for a new home. Grist is a spreadsheet with a database and app-builder flavor, and supports much of what Spreadsheet.com does well (including relations, attachments, formulas) and more.
They made a migration tool to help you move your data quickly, including attachments and relations, without the need to manually download and upload your files: Migrate from Spreadsheet.com - Grist.
To help you get started, Grist is happy to offer one, free 30-minute coaching call. Book a time from your Grist account.