If the FTP-SFTP conversions are mainly used for mainframe dataset transfers to z/OS hosts, and you want to to enable direct MVS dataset access on the server side, activate extended streaming by defining
SSH_SFTP_STREAMING_MODE=EXT as an environment variable. Deactivate checksum mode by defining also
SSH_SFTP_CHECKSUM_MODE=no as an environment variable. The checksums cannot be used with streaming, because checksum calculation requires staging, and then file transfers also use staging, and that excludes streaming.
Use the extended streaming only in connections to z/OS hosts, as it can slow down the transfer of small files in other environments.