I have a workbook with multiple sheets in it. Each sheet represents a different client and each sheet has the same column layout and data types. I need to build a summary page that combines information from every sheet together into one sheet but this appears to be impossible in Proton Sheets.

In Google Sheets the notation for this is dead simple:

={Sheet1!A:C; Sheet2!A:C}

The above usage is simple but array literals are much more powerful than that when combined with other functions and formulae. External resources:
https://www.benlcollins.com/spreadsheets/arrays-in-google-sheets/
https://davidmeindl.com/google-sheets-combine-multiple-ranges-sheets-into-a-single-range-sheet/