Getting a more stable signal on very long strips of adressable RGB LEDs
Rather than needing line buffers, I suspect that your string is just too long from a power point of view. The strip power and ground traces are simply too thin to carry 30 amps successfully.

I suggest that you find some way to access the power and ground traces (particularly the ground) at 3 meter intervals, and solder a 16 ga wire to each access point. Run the wires back to the power supply. Make sure you insulate the solder joints. At the very least you should be able to access the lines at each end of the strip, which would be better than nothing.

EDIT - With the inclusion of your scope traces, it's pretty clear that you're out of luck. The lower trace indicates that the chip has very slightly slower low-to-high propagation time then high-to-low. This means that, over many successive transmissions a positive pulse (or the positive portion of your clock) will gradually get narrower and narrower. As has happened to you.

But it's not the manufacturer's fault. Look at your numbers. With an 8 MHz clock, it's high for about 60 nsec. Your lower trace shows that your clock pulses have shrunk to almost nothing - call it 10 nsec. This means that you have a propagation delay imbalance of 50 nsec/1500, or 33 psec. Trust me, there's no (single-ended) process which can guarantee better performance.

For what it's worth, a reconnaissance satellite camera had similar problems which were traced to Altera FPGAs which had imbalances of several nsec, and the imbalance was stated in the data sheet. The cure was to invert the clock between each stage so that the imbalances cancelled each other out. Obviously, this isn't an option for you.

As I stated, I'm afraid you're out of luck. The manufacturers apparently never forsaw someone trying to extend their strips to the extent which you're doing.