Consequently, this SRFI suggests that Scheme's mutable strings be used only for buffers and other relatively short sequences of characters, while using the immutable texts defined by SRFI 135 for long sequences of characters.

But I'd like it to omit mentioning buffers.  String construction should be done with builder object---we have string ports for left-to-right construction, and vectors of characters for known-length, random-access construction.   The idiom of make-string and string-set! should be kept just to run legacy code (but maybe very inefficiently)