Here's how I would do it.
dropbox.com/scl/fi/pxjel9inotzuymmdaq7em/idlenumbersexample.c3p
An array (or other kind of table style data structure like .csv) for the labels makes it cleaner, more flexible, and more easily modifiable in my opinion.
Of course with any system like this you're going to run out of labels eventually. You can use the letters in sequential order as I've seen many idle games go (a -> z -> aa -> az -> ba -> bz ect.), or use a gameplay reason (a wall) to force resets every once in a while like Idle Champions of the Forgotten Realms. Or you can just use proper scientific notation 10^n. It would be great if everyone used and learned the scientific notation style to display large numbers.
Edit: There are still some potential improvements to be made, such as having a fallback display method after the number gets too large for defined labels, and zeropadding decimals. Perhaps also a variable to specify the number of significant figures desired.