I've used Forms to import grades into Canvas often. I don't know if things have changed if you convert them to a quiz.
Here's what I do using Excel. I haven't tried it using only Chromebook and Google Sheets but it should work.
- I have a master workbook called gradebook that is just the download of the Canvas gradebook, with all columns past column F (Section at our school) deleted.
- I copy the grades from each Google Sheet linked to the form into my workbook, like this. (This one is for homework, since students turn in their assignments using a Google form)
- I create a column in the Gradebook using the column name that Canvas wants and use a VLOOKUP formula to put the percentage into a column (as below).
- Saving the gradebook sheet as a CSV file saves only the one page and it can be uploaded directly to canvas.
Here's a picture of one of my spreadsheets this semester.

This sounds like a lot of work, but, in fact it takes me less than 5 minutes, from start to finish, to upload the student's grades for each assignment.
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