LED-driver board with WS2818/WS2811 and PT4115
I like your design. A lot.  I have a similar design in progress but 50V rather than 12V. And a TI TLC5973 instead of WS2811 and LM3414HV driver. I looked at this as if I were buying them myself.   

My main concern is it appears the thermal pad on the PT4115 is not used.  You may (not sure) have to dissipate up to 0.5W x 3. I put the driver near the edge of the board and ran the thermal pad to a screw hole in case a heatsink is needed.  There is a good chance you will not need to do anything as this design appears fairly efficient. I do not know if you have used this chip and have measured the temperature.  

Footprint for LM3414 Driver
 






  I'm especially wondering about the current sense loop and if it's okay
  to share one part of it with the power supply loop, or if I should
  have individual traces?


The current sense is just measuring the voltage across the sense resistors.  You should have no problem there. 





I also had a design with jumpers. I used 3M jumpers and they were expensive and a pain in the ass to use.  I switched to a DIP switch, much happier.  





Do you think it is possible not to use the transistors and zeners on the WS2811's outputs? The DIM high level voltage range is 2.5v to 6V and the frequency is low.  But you can always short out the transistor. But I am not familiar with the PT4115.   





What is "JLCPCBs special offer (2*3*10)"
Have you used JLCPCB previously? 





Who is going to do the assembly?  

If just buying the PCBs I would have no problem pulling the trigger on this design. But I go with the philosophy of slapping something together quick and test and debug rather than taking the time to go over things meticulously.
I typically do 3-4 different boards at a time to save on shipping costs.
I like to build a prototype before assembly.