I know this is sort of the ‘wrong’ answer, but I would suggest taking advantage of a deal like:ebay.de/itm/385476659762 That’s a rephone, new condition, for 149 euro. If you can afford it, it has a replaceable battery and 6GB of Ram. The default install has 5+ GB of space. It’s a lot less hassle and it’s a fast phone. Sadly, even all that RAM doesn’t solve the bombing browser problem.
I’m still happy to use my original Volla for development only and I keep it at 4.4 (and a GS290 at 3.4) but that’s for testing and development purposes. Daily driver is the rephone (GS5 backup).