creating a script for Numbers (and assigning key combinations)

tell application "Numbers"
	tell front document
		tell active sheet
			tell (first table whose selection range's class is range)
				repeat with c in (get selection range)'s cells
					tell c
						if formula is missing value then
							set value to "=" & value
							set format to automatic
						end if
					end tell
				end repeat
			end tell
		end tell
	end tell
end tell

Create a service from that script using the Automator app.

1. Create a "Quick Action" with no input in application Numbers
2. Use the Run AppleScript action
3. Delete the default script and paste in the code above (95% of which was from SGIII's script)
4. Save it with a suitable name.

It will show up in the Numbers->Services menu. You can give it a keyboard shortcut in System Preferences->Keyboard->Shortcuts->App Shortcuts (or whatever the equivalent location is in more recent MacOS versions). When making the keyboard shortcut you need to type the name of the service in exactly as it appears in the menu in Numbers. Capitalization matters. Option is typically used for optional characters on the keyboard (Option z is Ω). You can steal Command = from Format->Font->Bigger.

Select a cell or a contiguous range of cells then run the service.

"A script for Numbers that selects all the contents of a cell, copies it and pastes it in the form of a formula" says that there is one text-formatted "formula" in each cell that needs to be converted (I assume in the same cell). "I have around 20,000 formulas in text format inside the cell" says that there are 20,000 formulas in one cell. These are very different problems.

If it is one text-formatted formula per cell and if they all start with an = then all you need to do is change the cell formats to automatic. I don't think there is a simple way to make a keyboard shortcut for that but you can select all 20,000 cells and apply the automatic format to all of them at once.

If there are 20,000 "formulas" in one cell and if they are separated by a line feed and if they all start with an =,

1. Select all the text in the cell
2. Copy
3. It might be necessary to enlarge your table so it has all the cells (cell addresses) that these formulas reference. Otherwise you might get reference errors when they become formulas
4. For the column that is going to get all these formulas, format it as "automatic".
5. Click on the cell where you want to start pasting your formulas (don't double click, just a single click to select the cell).
6. Paste

They should paste into 20,000 cells in the column, all as formulas.

It will be under the Numbers menu, not the Edit (Modifica) menu.

Numbers->Services

If the keyboard shortcut is not working, make sure that the menu title you entered there is exactly the same as it appears in the Numbers->Services menu, character by character. Any deviation at all and it will not work. The letter "a" is not the same as "A", for example.