I use okular on Linux Mint.
Unfortunately the review bar which contains the annotation tools has to be manually loaded up each time okular is opened.
This is outrageously annoying if you open many hundred documents a day you need to highlight!
I neither found any option to show the review bar by default in the program's settings or via web search. I did not even found a shortcut that could be bound to a mouse key for easier opening. (The standard F6 shortcut does not do the trick for me.)
Is there any solution you can recommend?
Based on the information available on the official KDE Okular Handbook (currently at version 0.26) there is no functionality associated with this feature, so it technically can't be done by default from inside Okular.
To access the "Review" function (which includes highlighters, etc) you should press F6 or choose the Review option from the tools menu. At the moment all you can do in regards to tool configuration is change the available components of the review tool, adding or removing highlighter/marker tools. You can also configure shortcuts for the Review function, assigning an additional/alternate shortcut to open the review tool panel (Relevant to those with multimedia keys associated with their F-keys).
If the lack of this feature bugs you, you can always post a feature request to the KDE Bug Traker; Remember to search for existing requests before posting to avoid duplicates.