"1) As the trimmer will be used by fit and less fit individuals, weight is of importance, something that is not too heavy is preferred, hopefully while not sacrificing on performance"
This perfectly describes my wife and I, she being the fit one. She just turned 60 and can run our Echo SRM210 for 4+ hours in 95-degree heat and do it again the next day. This is cutting fence rows, under trees, and pond embankments. Once you get the feel for the trimmer they are really not that heavy, if you are healthy enough to work in the dust and heat the trimmer is a secondary consideration.
I have used harnesses and t-handles and find them cumbersome unless you are running a saw blade and than they are necessary for control. Based on the pics of your weeds I do not think you will be happy with a battery operated trimmer.