I've done this twice accidentally, both times when I've said yes to save the project before quitting but it doesn't always work so seeking a little clarification on how to actually do this when I want please.

I was using v18.04.1 and I am now using v18.12.1 and all my defaults have gone.

What I need to do is be able to change the default "render to" directory, quality of the video from 23 to 25 and also the default frame size from 180 to 540p with this resize enabled.

For your last 2 issues, you just need to go to the kdenlive settings and change what you want in "project defaults". You can also change the directory but i'm having issues with that so i can't be much help.