For such a large batch of pictures to convert, I would use IrfanView because of its batch-converter and the lots of options it has to fine-tune the conversion. As algorithm to convert the pics with, I would use most probably Lanczos3. IrfanView also works under Linux with WINE, I guess that it will run on macOS with Winebottler too. If you want to use IrfanView, do yourself a favor and download its plugin-pack too, it’s worth it if you handle seldom formats.
Next software with a versatile batch-converter is XnView, it is available for Linux, macOS and Windows, has even more options than IrfanView I believe, and offers the standalone converter XnConvert too, additionally to the one that it is built in into the main-software, but my favorite is IrfanView, because of the interface.
Another good software to reduce picture sizes, in memory-space used, is RIOT (Radical Image Optimization Tool) under Windows.
But the batch-converter of RIOT can also adjust the dimensions of the pictures to process (and you can hand over a single picture to it, that’s handy because the single picture-mode of the main software offers only to resize dimensions if these are above a certain size), but its main capacity is to reduce the file size, and there are both, main program and batch-converter, versatile and effective, it works with JPG’s, GIF’s and PNG’s and offers some options and has a preview. It sometimes offers enormous file size-reduction, but always good file size-reduction.
Here I can only guess that it will work under Linux and macOS with help of WINE or Winebottler.
All the programs above are free of charge for non-commercial use, and reasonably priced for commercial use.
Michelist