Recursive LAMBDA() function to create a formula that adds internal separators to a string in Excel
I thought about this for a while and came up with some (what I think are) improvements:
=LAMBDA(base,width,paddingStr,TEXTJOIN(paddingStr,,MID(base,SEQUENCE((LEN(base)/width)+1,,1,width),width)))

This would:

No longer require a recursive LAMBDA() which should prove to be faster (untested)
Uses TEXTJOIN() which can handle errors and empty values internally so there is no longer any worry about that.
The use of MID() negates the use of LEFT(), RIGHT() etc.
No longer be using LET() which may also be using internal memory.