I typically leave the numbered value like "sketch001" and append to it something like "sketch001_bracket". This way the original sketch number is there as a reference in case I need to re-attach the sketch, I just ignore the number.
As far as asking for renumbering, FreeCAD pulls together many different programs , you would really be asking all of them to change their numbering scheme. Granted starting from zero can take some effort but FreeCAD allows any programmer access so the interface kind of reflects that concept of starting at zero.
After counting from zero for 35 years I just accept that I have 9 toes, 0 thru 9.