Just use strtol or strtof

Which of these is more likely to have bugs?

    int i;
    
    if (PARSENUM(&i, s, 0, 1000))
        err("Invalid input: %s", s);

or

    int i;
    long l; // need a temporary long to avoid overflow
    char * ep;
    
    errno = 0;
    l = strtol(s, &ep, 0);
    if ((ep == s) || (*ep = '\0'))  // make sure we parsed a number and don't have trailing garbage
        errno = EINVAL;
    if ((l < 0) || (l > 1000))
        errno = ERANGE;
    if (errno)
        err("Invalid input: %s", s);
    i = l;

You build these esoteric systems that use heaps of unmaintainable, unreadable code

Not at all. The point is to have self-contained routines which Just Work in order to ensure that the rest of the code is easier to read and maintain.