Google sheets macro for autofilling values - excel

Question

I'm working on a spreadsheet which without macros would require a couple of million cells containing formulas, I'd like a macro which would take the formula in the top row of a selection, and then copy/autofill formula in to rows further down the sheet, and then replace those rows with values once the formulas have calculated.
As there are about 20,000 rows with ~20 columns, and several sheets which are like this, even if I ran a macro which would copy the formulas down through every sheet and then replace them with values, it would still be very laggy, so I'd like it to only copy the formulas in to rows where a cell in a specific column isn't empty (there is an input cell in each row), as well as only copying to rows where the cells to be filled are empty.
If we take the example of a simple setup of column 1 being for input, and column 2 being a formula based on it, and I had already run this after entering inputs up to the second row, and then entered stuff into the 3rd and 4th row too, with something that looked like this. I'd want the macro when run to copy the formulas in to just row 3 and 4 cells, then replace them with values.
I'm also uncertain how I could get formulas in non adjacent columns to be copied down through the same rows, as they can't be shown in one range.
Right now I have 2 basic macros, one which would copy formulas down through a set range, and one which would replace it with values, as when I had them in the same macro it seemed that the replacement would occur before the formulas had loaded, so it would be replaced with nothing.
If what I'm aiming for isn't possible with google sheets macros, but is possible with VBA in excel, I'm not ruling out the possibility I'll have to try and switch to excel, but would much prefer doing it in google sheets.
Thanks in advance for any help.
Edit: This is the very basic setup I have right now:
function Autofillformulas() {
var spreadsheet = SpreadsheetApp.getActive();
spreadsheet.getRange('A4:C5000').activate();
spreadsheet.getRange('A4:C4').copyTo(spreadsheet.getActiveRange(), SpreadsheetApp.CopyPasteType.PASTE_NORMAL, false);
};
function Autofillvalues2() {
var spreadsheet = SpreadsheetApp.getActive();
spreadsheet.getRange('A5:C5000').activate();
spreadsheet.getRange('A5:C5000').copyTo(spreadsheet.getActiveRange(), SpreadsheetApp.CopyPasteType.PASTE_VALUES, false);
};
Edit 2: Thanks to comments I've now got it in to one macro:
function Autofillformulas() {
var spreadsheet = SpreadsheetApp.getActive();
spreadsheet.getRange('A4:C5000').activate();
spreadsheet.getRange('A4:C4').copyTo(spreadsheet.getActiveRange(), SpreadsheetApp.CopyPasteType.PASTE_NORMAL, false);
SpreadsheetApp.flush()
var spreadsheet = SpreadsheetApp.getActive();
spreadsheet.getRange('A5:C5000').activate();
spreadsheet.getRange('A5:C5000').copyTo(spreadsheet.getActiveRange(), SpreadsheetApp.CopyPasteType.PASTE_VALUES, false);
};