This is great work.
My own project is on hold primarily because it was to be installed on the roof of the under-construction garage. However because of a dispute with the contractor that project has stalled. As soon as that part is ready I will move in that direction. The reason is that my house is in a rural area. The family has decided that we want to live off-grid initially to see whether it is possible to do so. So having a very reliable system is important for you- perhaps much more than households like yours, who are grid connected.
I didn't quite understand the non-linearity part of the reason for changing the cross joint. Since you are using an off the shelf system to drive your linear actuators, is that the cause of the problem or is it some mechanical issue of the cross. In my view since you are using solar-cells or some kind of a sun tracker, the non-linearity, if any, should be ironed out of the system automatically.
On the batteries, particularly imported ones, one is not sure about the life the battery has already spent on the shelf. Pakistani importers are always looking for bargains as Pk is a highly price concious market with the consumer completely clueless about technical matters particularly in these cutting edge areas. So you may get saddled with a battery which has already sat on a shelf for over 5-6 months. Since these are sealed units one cannot even take a peek inside to confirm if the battery has sulfated or so forth.
I get my batteries directly from the factory which are taken off the assembly line and readied for me. However this is a unique case and not possible for everyone. Therefore I strongly suggest that you buy normal flat plate batteries which are anyway fmcg. Alternatively buy the volta tall tubulars which are deep cycle and would last longer if used with care.