Copying to first empty row?

I finally have the copying aspects of my macro working, after much help here thanks. To discuss the next stage I've attached a much simplified version of both source and destination workbooks used.

The macro currently copies data from Source-Simplified.xlsm to fixed row 8 of WalkIndex-Extract.xlsm

Sub Extract_Simplest()

Dim wi As Workbook
Set wi = Workbooks("WalkIndex-Extract.xlsm")

Dim wb As Workbook
Set wb = Workbooks("Source-Simplified.xlsm")

'Track name
wb.Worksheets("Track Data").Range("B5").Copy _
Workbooks("WalkIndex-Extract.xlsm").Sheets("Target").Range("C8")
  
'Main FW Heading (C6 & B3 in formula)
wb.Worksheets("Track Data").Range("B27").Copy
wi.Sheets("Target").Range("D8").PasteSpecial Paste:=xlPasteValues

End Sub

I want to edit that to copy instead to the first row in which col A is empty, row 4 in my example. The first run would therefore enter two cells into row 4, instead of row 8. And so on, as the number of rows increases.

This code finds the first empty row OK.

Sub FirstEmptyRowA1()
'Find the first empty row in Column A
Dim lngLastRow As Long
lngLastRow = Cells(Rows.Count, "A").End(xlUp).Row

Dim FirstEmpty As Long
FirstEmpty = lngLastRow + 1

MsgBox (FirstEmpty)

End Sub

But attempts to use 'FirstEmpty' in Sub Extract_Simplest() have so far defeated me and any help would be appreciated please