It is a launch only item so if you wait you won't get it. As far as I know there is no way to get the English version of the artbook except for this one week (I'm guessing the Japanese version will still be in print as an actual book but they are expensive). A frustrating practice. At least this time GOG sent out the notices correctly, some of the previous times they did this GOG didn't even notify people waiting for the game until after the launch items were no longer available. It doesn't look like the highest price version has any extra launch only content just all the DLCs.

Is there a place to get lossless digital versions of the soundtrack? Falcom will ship CDs but they are expensive.

Did they actually say this for the artbook as they usually place a caveat(I saw no notice)? I mean it is specifically listed as the DX edition and who would want to buy that DX edition after if they remove all that? After all, then it is just lazy cosmetic junk they made and NOT befitting of the DX title.
Yes on Japanese audio storefronts selling digital you will find FULL Falcom OST's. These are Japanese sites but if you find a way to buy them there I won't cry as NIS is making NO effort to sell them in these packages. If they were I would say nothing.

They don't seem to say that explicitly but it is what happened previously (e.g. Reverie had an artbook at launch that is no longer available) so I suspect that will be the case again, particularly since the title on the preorder includes the word Launch which they wouldn't do if it would be the same later. I can't recommend supporting such practices but if you really want it now may be the only time you can get it. Thanks for the hint about Japanese digital audio stores :).