You could also try my Plug-in:
https://forum.audacityteam.org/t/arrange-your-tracks-with-chain-it-up-ny/26201/1
You import the files via the import menu.
Select all tracks and start the plug-in.
For a pause of 300 ms just enter 0.3 in the gap control.
Once the tracks are arranged, you can mix them down or simply export which will do the same.