The ST-60 series of instruments, and that includes the "graphic" repeater - do not output any SEATLK to NMEA conversions whatsoever, The internal bridge is Seatlk it does some NMEA outputs but as you discovered limted.
If you want to interface NMEA without a display unit such as the C-70, C-80, or G series then you need this:
"When used with
Raymarine's RayTech navigation software, the E85001 enables direct SeaTalk communication between the PC and network, supporting functions such as active route and waypoint transfer. Alternatively, the E85001 can convert SeaTalk datastreams to NMEA 0183, and NMEA 0183 sentences to SeaTalk, enabling other NMEA 0183 compatible navigation devices to communicate with
Raymarine SeaTalk compatible instruments,
autopilots,
radars, and navigation aids. The E86001 serial data cable (sold separately) is recommended for connecting PCs to this device."
It can be had for around $150 - setup is a cinch and works well with displays or when building a device only solution that requires interfacing between other standalone NMEA products that you need to talk to seatalk.