Use variable in row reference in formula in Google Sheets

This is my sheet: (shared publicly)

https://docs.google.com/spreadsheets/d/1Pz5dkSHwSXne6f_tMoBJ1Fu3Z6JsvjTQgJZhHhpjia0/edit?usp=sharing

In the "Flatten" tab on that sheet, in A1 I have a formula:

={ARRAYFORMULA(split(flatten('Form Responses 1'!A2:A6&"|"&Rates!B1:E1&"|"&Rates!B2:E2&"|"&{'Form Responses 1'!B2:E6}),"|"))}

In that formula, I have to manually change A6 to A7, and E6 to E7 or A6 to A8, and E6 to E8 every time the rows in 'Form Responses 1' changes. As you can see, right now there are 8 rows of data, so I need to manually change 6 to 8.I figured I can get the max number of rows thus:

=MAX(FILTER(ROW('Form Responses 1'!A:A),'Form Responses 1'!A:A<>""))

I put this formula in K1, which shows 8 right now, and I can concatenate the string I want thus: =CONCATENATE("A",K1) to get 'A8' or =CONCATENATE("E",K1) to get 'E8'.

So, I tried:

={ARRAYFORMULA(split(flatten('Form Responses 1'!A2:CONCATENATE("A",K1)&"|"&Rates!B1:E1&"|"&Rates!B2:E2&"|"&{'Form Responses 1'!B2:CONCATENATE("E",K1)}),"|"))}

But I get the error: 'Argument must be a range'. What would be the correct syntax?

Thanks.