As a business user sheet would be a very nice feature to switch away from google.

Yes, spreadsheets would be tremendously useful, and would entice me away from Google Drive altogether, I think. (I bet it's on the cards.....) 🙏

I need it to stop using Google

Like NVR3JCT, Google sheets (and docs) are the last reason I have to stay with google.
I have dozens of small sheets for quick calculations and can edit them reasonably well on mobile and desktop. That is an indespensable feature for me. Cryptpad offers a hole spreadsheet app that looks extensive in functionality, but is unusable on mobile. To be able to quickly input numbers into a spreadsheet when on the go seems to be only possible with google sheets, at least not without some form of subscription.
If proton would offer such a spreadsheet editing functionality, I would transfer my google files immediately and get a paid account.

Literally the last thing I need to completely remove myself from the Google ecosystem. Just basic column/row summing would be fine.

We do NEED this. Basic Documents, Spreadsheets & maybe some sort of Slides copy like Google Docs or Microsoft Office are a must if you plan to store your info on a cloud.

I would second Clayton's idea to not just make it like Google Sheets/excel but Make it like Airtable. I have used Airtable for years and the constantly are cutting back on access and features and make their payment models ridiculous. I think Proton would get a lot of business from Airtable like features.
Doublely better if you can make a Spreadsheet and Database toggle when setting up a Table, but that might be too much.

I hope this becomes a thing especially with the partnership with Standard Notes. They have a spreadsheet functionality already.

A spreadsheet editor with database features similar to Airtable

Like Google Sheet. Implement sheets in Proton.