So, first you need to find/make a script to automate proportionally filling the canvas with the layer content. Other options could be making a script or action stretching/distorting the layer to fit the canvas or content aware fill or content aware resize so that the layer fills the canvas.

Then this can be automated.

A search of the forum returned some topic titles that looked appropriate, however, I don't have time to check the content now:

How do I automatically scale a layer to fit a canv...

How Do I Write A Script Resizing A Layer To Canvas...

And a web search:

https://community.adobe.com/t5/photoshop-ecosystem-discussions/fit-layer-to-canvas-keeping-aspect-ratio/td-p/10519295

https://community.adobe.com/t5/photoshop-ecosystem-discussions/scale-layer-to-current-canvas-size-photoshop/m-p/5217315

https://community.adobe.com/t5/photoshop-ecosystem-discussions/automatically-resize-a-layer-to-fit-canvas/td-p/2218395

https://gist.github.com/jawinn/ab4df1c33d0743e41fd3

https://graphicdesign.stackexchange.com/questions/146988/how-to-write-a-photoshop-script-to-resize-the-image-until-it-covers-the-whole-ca

Thanks for that. I now have a script that fills the layer to the canvas and middle-centres for me. So I open my template of the relevant size, say 24x30 inches 300dpi, embed the original large image, run the script to fill the layer to the canvas, save a copy as a jpg, give it a new name reflecting the size. Then I start all over again, open 16x20 inches template (and repeat the process). Then open 8x10 inches template ... for as many sizes as required (usually about 7 or 8). So it's still a bit of work, especially when I work on multiple images (designs) each day. I created an action, but found I needed to create a different action for each image, because of the file saving and naming. I tried to put stops in the action so I could select the image file, and save it the way I want, but the stops would not work for me. If only IPP had the "fill to canvas" implemented, it would be so efficient to set everything up once, and do it all through IPP ... otherwise if you have more advice, I'd be happy to get some guidance.