It looks good. The problem for me, to use it is that Google Sheets and Docs are way more fully featured, even if they aren't integrated as neatly as you have done. You will need to find the people for who having calendars, docs and sheets in one place matters a lot. Maybe it is the kind of people who already use notion and obsidian etc.

There are some UX things I think should be looked at:

1. New sheet asks me 'Create source' or 'select source'. One is black and the other blue. I assume they are both links? Looks like I need to click "Create source" to do the thing I would intuitively expect, which is to show me a blank spreadsheet to edit.

2. First column of a new spreadsheet is called "Name" and I cannot rename it, it only has sort options, but if I add another column it is called "unnamed" and I can rename it. I am not sure why this is. Since all I have been told at this point is that it is a "sheet" I am expecting a free homogenous experience like Excel. If it is not that, you might think about how to make it more obvious what it is. (Do I need training, a quick tutorial, a quick highlight-feature/click-next intro etc.?)