How can I make a large set of travel photos smaller for sharing without noticeably hurting quality?

I think you're going about this the wrong way. If you have 2GB of images at 4-5MB each, that's somewhere between 400 and 500 images. That's way too many. Even your close friends probably don't want to wade through all of that.

Instead, go through and pick out the very best 10%. Or 5% or even 1%. Take some care and write a meaningful caption for each one. An album of 40-50 good, interesting photos is better than a big dump of half a thousand unsorted ones.

Choose images which show the story of your trip, and which present a unique point of view not available in postcards. You may find that with this reduced data load, you don't need to resize at all. Or, if you do, since you have fewer photos to work with, you can be more careful about selecting the best parameters for each one individually rather than mashing them all down in bulk.

I'd suggest an online web gallery service for sharing, rather than e-mail. Upload once and all your friends can look.

Best of all, by choosing the best images and not bothering with the others, you are literally increasing the average image quality and saving bandwidth and storage. It's a win all around!