What is the right strategy? video photo etc?

Search Stock's database for keywords that have little representation.

Start with diseases & viruses. Fill it.

If the quality is good, under-represented assets are more likely to be accepted than assets they don't need. 

You say that your assets are "generally unique". Have you searched the database for similar images using the keywords in your assets? With >800 million assets in the database, virtually ever topic imaginable has a lot of competition. It has become increasingly difficult to find subjects that aren't already  over-represented in the database, but that is what you must attempt to do.

Your sales may be low because your metadata is lacking quality. In all cases, you should continue to contribute without sacrificing quality, as that is the only way to increase your portfolio.