Newest firmwares on the 2630 and 2600 will take 7 decimal places now.  I have been doing it for 2 years.  I feed my 2600 from an FMX and Nav2 (coming out of the Nav2) for the purpose of yield monitoring in my combine (I have autopilot on a green machine.... I know)..  Reason for 6 decimals was because some reason "they" didn't want any better than 4" precision coming in from 3rd party gps which is 6 decimals.  7 decimals is roughly 1/2" (depending on your latitude).  Either someone griped enough for they figured it wasn't that big of a deal anymore.  I dunno.  https://en.wikipedia.org/wiki/Decimal_degrees  Anyways, 6 decimals (4") is plenty for yield mapping and such.

I'll probably jinx myself but I have 0 issues feeding NMEA into my 2600 from trimble.  Again, I am yield mapping only and not doing product control.

I agree, you don't need or even want VTG.  Turn that off and use only GGA, GSA, RMC all at 5Hz as CCSTX states.  Reason, for this is at 19,200 baud any more than those messages at 5 hz can get close to "saturating" the line and you run the potential for incomplete messages if either the sender or receiver buffer get full/overflow. In theory you can send that much information, but in practice if there's any hiccup in processing the incoming data, the receive buffer on the Deere side will overflow and it will appear to the CPU that the data is incomplete and do "wonkey" things.  Turn the VTG off and you should at least have a better shot.

I have been told that the 19,200 input is fixed on the Deere side.  I could have swore that I had tab where I could have changed it.  It's raining and the combine is at my uncle's otherwise I'd look.

Question is this:  Are you steering with the 750 as well?  I have seen some bugs on the early firmwares for ez-pilots do the same thing when coupled with a 750.  It does the same exact thing where it "twists" the gps output.    To me, it looked like the gyros (either the imd or the wheel angle sensor) weren't reporting or computed correctly.  Firmware update solved this.