Need help with excel formula to convert groups of letters to numbers.

Another approach would be

= REDUCE(string, letters, 
    LAMBDA(str, chr, 
      SUBSTITUTE(str, chr, XMATCH(chr, letters))
    )
  )

where 'letters' is the array of letters you are replacing in 'string'.

With FDAJ in A1:

=CONCAT(SEARCH(MID(A1, SEQUENCE(LEN(A1)), 1), "FREDAJVON"))