Workflow for Virtual choir project

Hello!

I'm working on a virtual choir project for my choir. Since we cannot meat because of the pandemic, we decided to try to record ourselves at home and mix everything in one video with all the voices and images. Our first project turned out quite nice (for an amateur choir and an amateur video editor), so we're working on a new one. Editing went fine, but I'd like so speed things up for this next project. My venerable MBPr did the job well, but the final render took 4-5 hours, during which I couldn't use it for anything else. 

The general idea woud be to start a AME or Handbrake job every night for a few days while I recieve the videos, so that editing and rendering is as fluid and fast as possible. 

Singers will use their phone or computer with integrated microphone to film and record themselves. I will recieve the files one at a time within a few days. Most files I got for the first project were H.264 (720p ou 1080p) at 29.97 FPS (VFR). Some files were in onther codecs (HEVC) or other frame rates (30, 30.04, etc.) Is it ok to assume that if I convert all the footage with the same preset that will be used for the final output, I will save time when rendering and outputing? 

In the final product, each video will always be the same size (320 x 180). There will always be 36 images at the sametime. Would I gain some time at the end if I change the image size (from 1920 x 1080 to 320 x 180) before I start edting?

Is there a preset I should use when I create my sequence? The final output is for Facebook and YouTube.

Thank you!