I don't know what life you can expect from gel cells under these conditions.
Gel are worse than VRLA for getting charged, the final 20% takes a very long time. If you are using them regularly as you seem to be doing then they are probably never fully charged and having a higher charging current available may not help much as the final part of the charge will be at low current even if you can speed up the bulk.
I am not sure how much you can raise the voltage for cyclic use, VRLA will let you raise it quite a bit above the normal float value. You can't equalise them in the same way as flooded cells but I think you can do it a bit for a short time.
I think VRLA would be better if you must have sealed batteries but I would think you would do far better with flooded ones. With flooded ones you may have to use calcium technology with a low charge rate to get them up to volts but that should never be an issue with gel. I don't think the low current in its own right is the problem as long as you have been able to get them up regularly and you say you can do this daily.
I would look at the individual units with a voltmeter and see if they are all the same or whether you have a duff cell in one unit that is messing up the whole string. If they are all equal then they are probably worn out.
Batteries are a pain and obviously you mainly need them for back up, if you cycle them all the time they won't last nearly as long as under float and standby conditions.
I would have hoped they would have done longer but I am not familiar with them, many of the solar sealed ones do have a good life but you don't half pay for it in the first place. If these were budget ones they may have run their course.
Flux