Hoping to have well understood your doubts, I would answer this way:
- The catalog (yes, that's the LR database) occupies a very small part of your disk space.
The "Previews" which reside under a subfolder (sub-directory) could make this unsignificant disk occupation of the database a little more important or not depending on how you set the "Preview" life.
- In any case your images have the lion's share of disk space.
I am just a "dilettante" and I have just a notebook with some external disk:
my database(s) is on the computer internal disk while all the images are on the external ones.
I have no doubts that the best way to start is that: catalog on the internal drive, images on the bigger external disks.
Eventually, in case future lack of space on computer internal disk, it will be very easy to move the catalog to the same external disk where the images reside or, even better, to some other *fast* external drive. Moving the catalog could be just a matter of less than 5 seconds if you give up the "Previews" and accept to rebuild them on the new drive or will need just some minutes in order to move the entire structure/substructure of the db+previews+backups.
Finally I would not split in different databases by date (months or years shoots), but yes I organize the actual images mainly by date (yy-mm-dd) *physically*, but everything is managed by one catalog db *logically*.