Remaking/simplifying the texture file from online photogrammetry file

Hi folks

This is my first post and I’m a very basic user. It wouldn’t surprise me if I was the most inexperienced user on this forum, but I’m keen to learn.

I’m using 2.8 to make simple tweaks to files created by the Display.Land app using my phone. The app lets you take photos around an object then creates quite a high quality 3d model of your scene. You can then quickly crop and edit the file on your phone. However, when you download your model it comes uncropped and quite inefficient in terms of file size.
I’m trying to replicate a process that I dabbled with a few years ago where I used a phone app called 123d Catch by Autodesk, to scan objects. I then worked out how to use blender to trim the object, simplify the polygons and save a trimmed down texture file.

Several years down the line I am really struggling to do the same.

I can import the .obj or .gltf files and crop them, but after that I’m stuck. My texture file that Display.Land creates is 80% blank (with the sampled photos just forming a strip at the top of the image and the rest being black) and therefore a big, unwieldy waste of data.

I was hoping that someone could describe the steps to make my texture file much smaller with all the elements filling the whole image file. I seem to remember that I used to bake and unwrap, but I can’t really remember.

On another note, I’m having no luck decimating the polygons. It just leaves the texture on the model full of black gaps. (However, I’m happy using 3d builder to simplify the geometry in a couple of clicks).

If you’ve got this far - thanks for reading. I hope that someone has the patience to give me a hand.