Export settings?

1.  ....... but my main concern is outputing a file that is good quality to save on my PC. 

 

2.  Also, I've been shooting in 4K and downsampling to 1080 on export.  Not sure if that is really worth it, or should I just shoot at 1080 to begin with.  .....

By @xrayzzz

My practice is to shoot in 4K and produce HD 1920x1080 videos.  The HD videos display well on my 55 inch TV, computer, YouTube and Vimeo.    

1.  Good quality on your PC depends on your monitor.   Most do very well with 1920x1080.   Yours may be different and you may end up saving your work in 4K.    

2.  The primary reason to shoot 4K is for the extra opportunity in post processing to do things like "Pan and Zoom" or changing the scale ("cropping").  If you don't intend on doing that, you will have more manageable file sizes and easier processing with 1920x1080.

You're trying too hard. You can output a file that optimal for use on YouTube simply by going to Export & Share/Device/Computer 1920x1080 MP4. There's no need to create a custom output setting.