I agree with Patrick - Use what works for you. There are many ways to approach this, and all have merits.
In my case, I did NOT use DigiKam or any other photo catalog system. The collection of photos that I scanned will be sent out on USB sticks to many family members, none of whom use DigiKam. Therefore it was imperative for me to label photos in a way that would stick with the file no matter what computer system or image viewer was used. I wanted a method which can be seen on almost all image viewers and operating systems. DigiKam is a fantastic application - and about as portable as a nuclear power plant.
EXIF data fulfills that need because it is embedded in the file. It is not an auxiliary file or index. It is completely independent of whatever application is used to view and organize images.
One major disadvantage to using EXIF data fields is they are not easily searchable. I cannot, for example, search all my photos for Susan Gladys Cundiff. It is possible to write a script which would open each file, extract the Comment field and run a text scan on it. That would be VERY slow. So far I have had little need to do this.
Exiftool has two features that I used a lot. First, it can tag multiple files in one go. Second, it can copy fields within a file. I used the Comment, UserComment and Caption standard fields. I first put my notes in the Comment field, then used Exiftool to copy that to the caption and UserComment fields. Another feature which I used only once is to create a text file of tags, then apply them to a group of photos.
It is worth noting that another Linux tool - exiv2 - can do all of the same things that exiftool can do. I found some GUI tools for editing EXIF data, but they were very hard to use. The EXIF field editing feature was buried many layers down in menus. Command-line tools were much easier to use.
When I assigned names to the people in the photos, I used right-to-left, back-to-front sequence as much as possible. When that was not clear in the photo, I expanded the note to try and make it clear. I always used maiden names in parenthesis, which makes it easier to find people in databases.
My photos are organized in directories and sub-directories, either by base family name or by event. I have, for example, a bunch of Christmas directories which are named similar to “1995-Christmas-AnnArbor” or “1997-Christmas-GrandIsland”. There are directories for massive family gatherings. I mentioned the flip calendars that my nieces made. Those go in their own directory. I wound up using only three levels of directories.
For file format, I used JPG as much as possible. Dealing with TIFF would have taken more time than I wanted to spend, and the scanners I have are not that good. Correspondence which can be run through OCR is saved as PDF/A format with the text embedded. I transcribed a few handwritten letters. The scanned image is in a PDF with a TXT file embedded. Most of the handwritten letters remain as images.
When I used photos or letters in Gramps, I made a hard link from the main photo directories over to the Gramps media directory. Gramps is perfectly happy with this. Symbolic links work too.