Thank you so much for the answers :). I'll answer separately for regular and dvddisaster archiving from now on.
@Everybody who replied: I have burned the DVD! There's no problem. It works fast and OK. I can't see any difference to a normal DVD+R with 1, 3, 10 or 20 files on it. So that's really great so far. The disc is also a great burn with PIF max. of 1 and PIE max of 4 after an ECC1 scan. So if nobody sees any problem in that huge and complex FAT for data life security, I'm a huge step forward now.
@Hemispasm: Yes, exactly. Even if the zip file had no compression at all, a little read error would effect the unzipping functionality of the whole data or bigger parts of the whole data. The only reason I thought about making an (uncompressed) zip was the huge and complex FAT.
@DrageMester: Thanks for the warning :). The files are in many folders and there are many deeper folders in many outer folders again. I guess the problem you mentioned didn't occur because of that. But it's a very interesting point you mention. Just to know it for eventual future cases: If the 20,000 files would ALL be in the root directory ALONE, would it be a bigger risk for data life as well or just an annoying but otherwise harmless problem?
@mciahel: Well, this is a very interesting thing you mention there. That I already burned one disc, does not at all mean, that I won't do that dvdisaster disc as well. I will look closer what that actually is later today. I'm very thankful for your link, since this seems to be a great data life security solution for all my data, not just this current disc. I will come back and report.