Not the original poster, but if I were doing it with find rather than fd (or ag or rg, which I think can do it without fd or find) I'd do (untested)

    (find "$projroot" -name .git -prune -o -name node_modules -prune -o \
         \( -name '*.[tj]s' -o -name '*.[tj]sx' \) -print0 ;
     find "$projroot"/node_modules \
         \( -name '*.[tj]s' -o -name '*.[tj]sx' \) -print0 ;
     find "$projroot" -name .git -prune -o -name dist -prune -o \
         \( -name '*.py' \) -print0 ) |
    xargs -0 grep /dev/null "$search_pattern"

And, although that approach does work (and maybe even that script will work without any bug fixes) it probably goes a substantial distance toward showing why fd was written.

Doing the three searches separately and exiting after one succeeds is only slightly more code.