I have over 35,000 images (and growing) so I feel your pain! I had the same problem.
I need all my product images to be 600 x 600 and my sub-category images to be 200 x 200
There’s something within woocommerce that will set your image sizes to what you specify but I found it make them out of proportion so I do it manually.
I crop each image just using Windows Photos and try to make them as square as I can, but it’s not always possible. If I can make them square it’s much easier. Then, regardless of what size they are I use a free program called PIXresizer that allows me to make the largest of the sizes 600. So I could end up with all my images 600 x 600 if they were square, or if they weren’t able to be cropped they could be say 600 x 400.
Then the difficult part. And this took a lot of googling around for us to work out how to do it. We created an automated batch program within Photoshop that will take the Landscape size above of 600 x 400 and make it 600 x 600 by putting a white border around it.
There are other programs you can use if you don’t have Photoshop, but I don’t know how to use them.
We worked out it took what would’ve been 4 1/2 hours of resizing photos manually down to 3 minutes.
Hope that helps