I don't think shorter videos will necessarily increase retention, views yes but not retention and one reason for that is 10 min watched time of a 30 min video is the same as 10 min watched time of a 10 min video.
My videos are mostly 30 min and have an average watch time around 9 min. So you should get atleast that.
just look at your analytics on every video and see when they leave. If all of them leave before the 5 min mark then you have to work on your content. They are simply not interested enough to watch it.
but if some of them stick to the end of the video then you might instead have a Title problem. If you have missleading titles and people think they will see something else they will leave early and kill your retention. Misleading titles might give views but low retention. (I haven't watched your titles so I don't know how they are)
like this, if 1 viewer watch for 30 min and 1 viewer leaves after 30 sec. then your total watch time will be 15 min
1 for 30 min and 4 for 30 sec, total watch time 6 min
1 for 30 min and 10 for 30 sec, total watch time 3 min.
and if you shorten your videos
1 for 10 min and 1 for 30 sec, total watch time 5 min
1 for 10 min and 4 for 30 sec, total watch time 3 min
1 for 10 min and 10 for 30 sec, total watch time 1,5 min.
short answer: thumbnails, titles, description and tags drive traffic (searches and suggested videos), content makes them stay (retention)