You have to understand that smaller jpeg file size is often a GOOD thing, not a bad thing.
https://www.damiensymonds.net/2015/08/about-jpeg-file-size.html
Well, start here: https://www.damiensymonds.net/2010/05/best-practice-for-web-photos.html
I think you think this is more complicated than it really is.
Just resize using Image Size, then sharpen and save.
Well, none of that is any different with your R5 than it was with your previous cameras. It's ALWAYS been necessary to shrink files for email and Facebook.
How have you done it in the past?