Make a cell Mirror another when not Blank, Editable otherwise

Using Google Sheets

I want cell C1 to be editable via free text, but if cell A1 is filled, I want cell C1 to instead display what cell A1 has.

Basically I want something akin to the functionality of

=If(isblank(A1), [Allow this cell to be editable], A1)

But where actually editting the cell doesn't make me have to reinput the formula again

I've been trying with conditional formatting and Data Validation tools, but I can't seem to both make C1 editable and mirror A1 conditionally without one breaking the other

The other option I can think of is having another column and parsing it that way. So doing something like

=If(isblank(A1), B1, A1)

And locking C1 out of being editable, but that uses a lot more space in the sheet.

Thanks