No problem, I always want to check with the troubleshooting guide first since it's quicker than me typing out everything first.
Pi 3B+ is not usually one that has an issue with the OS config, it is often Pi4.
With the longer wires, I have had users tell me putting a resistor on the line can help, but I don't think 50cm should be too long - maybe when you get closer to 100cm. The level shifter may help, but this depends heavily on the specific LEDs. If your buck converter can be adjusted, try and drop the voltage lower, since that will make the signal from the Pi closer to what the strip expects.
The maths for this is simple, the strips (to spec) want minimum 0.7*VDD for signal, so 5V this is 3.5V - above what the Pi does. However, some strips don't like 3.3V. If you drop the input voltage to slightly below 5V, then it should reduce the required signal voltage.
I assume the OS configuration test passes? It should be straightforward on a Pi3, it just needs
/boot/config.txt - if there's any
core_freq_min in there, try and take it out.