How to optimize multiple layer/image video edit?

I am trying to condense these 6 different layers of images. In the video below, you’ll see that I have created placeholders for images that will swap out throughout the video. I don’t know much about how exporting works, but when I have previously exported a video with multiple layers of images, it is usually WAY bigger than I need it to be, and I believe it is because those images are being written for every frame. These images are mostly static, only changing a few times per video, if at all.

So my question is, is there a way to save this placeholder as something that only lasts for a frame, but persists until it is changed with and edit? Or is there a way to optimize this? It took me two days of working on it just to figure out how to make the placeholders (very proud of myself for that). I don’t know the jargon and barely know what I am doing. Most of this has been trial and error as I don’t even know what what I am trying to do is called. Please respond with steps if possible because I don’t know the whosit from the whatsit, and layman terms would be much appreciated. Thank you!

Edit: I realize I didn’t give much info on how I got to this point. So made each placeholder image a “set” with the size, position, and cropping I needed on the layers below the main screen layer. Then, on the main screen layer, i added a chromakey: simple to the green boxes where the images go. I have it now that I can replace any of those placeholder images with the image I want, and only need to move it around slightly to get the centering correct.

Well, I messed around with it until I found out a solution. Whether it is the best solution or not, it will work. What I did was I saved the above edit as an MLT, then did an Export Frame. It condensed all of the images as one image. Then I can upload that frame capture to my video, and only have it take up one layer instead of 6. I can keep the MLT up to date with what images are needed just by saving it and making sure to export frame instead of export as a video.