This is the question that spurred my most recent post on using a Google Script to allow multi-select behavior in a Google Sheet. With this innovation, end users can clean data in a spreadsheet environment (including MSPLs), rendering valid data in the appropriate format for upload. I'd love to hear what you think! https://thedataarealright.blog/2021/11/03/how-to-populate-multi-select-picklists-in-google-sheets-and-salesforce/