OK, here are two possibilities of what I might do. Note that a one-directional arrow represents a pair of wires for a half-duplex NMEA connection. Bi-directional arrows are Seatalk cables.
"Plan A" is what I attempted to describe above. If it works, it would interface depth, DSC, and AIS all to the chart plotter through Bluetooth, and also send GPS coordinates to the ST4000+ autopilot and radio. Note that there are two ways to send GPS to the autopilot connection: 1) wire it into Seatalk through the B&B baud converter and E85001 Seatalk converters into the ST30 depth gauge or 2) take it through the baud rate converter to the autopilot's NMEA input terminals. I denoted these two sub-options with dashed arrows.
I don't see a problem here with AIS getting to the Bluetooth adapter, because it is connected directly without going through Seatalk. The potential problem I see is that Seatalk may not pass through the GPS coordinates to the radio:
So if Plan A does not work, or I can't find the E85001 Seatalk converter for a price that I am willing to pay, this simplified "Plan B" arrangement should definitely work. It will not allow for displaying depth on my chart plotter, but the Seatalk cable between the depth and autopilot will allow display of depth on my Autopilot display panel. This is important because my view of the depth panel is frequently blocked by passengers in the cockpit, but the autopilot panel is much more conveniently located. It's a cleaner option that cutting a new hole and patching the old one to relocate the ST30 panel: