Are you using Windows 10? If setting -eres 0 doesn't work then there isn't much you can do but either upgrade 1 of your GPUs to 8GB and put it in the main slot. Or you can install Linux and use PhoneixMiner and you should be good for maybe up to 2 months or so.

I had the same issue as you, and couldn't get Claymore or PhoenixMiner to work under Windows 10. Switched to Linux and PhoenixMiner worked but Claymore still didn't. Was a pain to set up initially due to overclocking and undervolting settings but eventually got everything to work nice and stable.