You only put /bin/sh as the interpreter. You still used quite a number of Bourne Again shell constructs in the scripts themselves. Some are not in POSIX sh and utilities, some are not in the Bourne shell. (The Bourne shell is not conformant with POSIX sh, so targetting POSIX sh is not the same as targetting the Bourne shell.)
These include amongst others local, $(), >&-, the -a and -o operators to test, and command. (You also failed to guard against variables expanding to operators in the test command, but that is a common general error rather than a Bashism.)
Beware of testing against /bin/sh, even the BusyBox one, and assuming that that means POSIX compliance, let alone Bourne shell compatibility. Even the OpenBSD Korn shell running as /bin/sh or the Debian Almquist shell running as /bin/sh will silently admit some non-POSIX constructs.
These include amongst others local, $(), >&-, the -a and -o operators to test, and command. (You also failed to guard against variables expanding to operators in the test command, but that is a common general error rather than a Bashism.)
Beware of testing against /bin/sh, even the BusyBox one, and assuming that that means POSIX compliance, let alone Bourne shell compatibility. Even the OpenBSD Korn shell running as /bin/sh or the Debian Almquist shell running as /bin/sh will silently admit some non-POSIX constructs.