In the end, I came across a solution that I believe could be helpful to others in need.
Following AmacB's suggestion, I decided to download a customized version of the bootstrap css.
I then transferred the css into an excel sheet, with each rule on its own line.
Next, I added a x next to each rule in one column, and created a formula in the third column to display the rule if there was an x, and not show it if there wasn't. This modified list was saved and uploaded as a new css file for my site.
Although it worked, the resulting Css file was 32 kilobytes, too large to inline.
So, I started reviewing each rule to determine if it was necessary for above-the-fold content.
Any rules deemed unnecessary had the x removed, the updated list saved, uploaded, and tested. This process took some time, but eventually I managed to condense the css to only include essential rules for above the fold, totaling about 80 rules.
Afterwards, I included the following code in my php file header:
$TheCSS=file_get_contents('/css/bootstrap-reduced.css');
echo '<style>'.$TheCSS.'</style>';
As a result, my Pagespeed Insights score improved to 99/100.
However, adding 4kb to every page seemed counterproductive. So, after the page loads, I utilize jquery to link to the bootstrap cdn instead of inserting inline css on subsequent page loads.
Additionally, I developed VBA code to streamline the process:
Sub SaveAsTextFile()
TheFileName = "bootstrap-atf.css"
ThePath = "C:\Users\MyFolder\"
Sheets("Sheet1").Columns("C").Select
Selection.Copy
Sheets.Add.Name = "Temp"
Sheets("Temp").Select
Selection.PasteSpecial Paste:=xlPasteValues, Operation:=xlNone, SkipBlanks:=False, Transpose:=False
Sheets("Temp").Columns("A").Select
LastRow = ActiveSheet.UsedRange.SpecialCells(xlCellTypeLastCell).Row
Set Rng = Selection
Open ThePath & TheFileName For Output As #1
For i = 1 To LastRow
cellValue = Rng.Cells(i, 1).Value
If cellValue <> "" Then Print #1, cellValue
Next i
Close #1
ActiveWindow.SelectedSheets.Delete
End Sub
By marking desired rules in the excel sheet and clicking the "Save CSS" button (which can be added to the sheet), the rules are saved as a css file named bootstrap-atf.css (atf=above the fold).
This significantly simplifies the trial and error process.