Hi Michael,
Well, I know ThumbsPlus v7 almost inside-out, but I'm afraid that I can't really help you with 3d party databases.. I have very little experience with (and knowledge of) those. The best that I can do is usually to point people to information from the old newsgroup archives, but you already have those.
If you are sure that the database is approaching its limit (i.e., you have already removed all orphans and deleted records, you are using JPG compression and small thumbnails to keep it as small as possible), and you don't want to split it into multiple databases, then I guess a SQL-type database is your only option (or, with v8/v9 you could use external thumbnails). Personally, I would start by running a 3d party database locally, and only when that works well, I'd try to run it in a client-server configuration (just to keep things 'simple', so that when it doesn't work, it will be easier to locate the problem..).
As for the type of database, the newsgroup archives hold some good MySQL scripts that you could use with TP7, and I'm not so sure if there are any good ready-made Postgres scripts. As for TP7/TP9.. yes, please do thoroughly test v9 to see if it can do everything that you need reliably. I agree that the Python scripting option could be a real benefit, but for me personally, the stability and reliability of v7 (and its IPTC/EXIF editor) are still more important and that's why I still prefer that old version for my own uses.
I'm really sorry that I can't be of more help.. Best of luck!
-Daan-
ps: Since you speak German, you could also check the forums at
www.thumbsplus.de to see if there are still any MySQL/Postgres experts there.