Try the Ressurector cartos. They are 1.7ohm and hold alittle over 2ml of juice so they last a pretty long time before needing a refill. Personally I can't stand the CE2/CE3s. They do give good flavor, when they work.. But they are very finicky and I think it's annoying that you basically have to mod them just to get them to work like they should. The Ressurectors are very consistant and give an awesome vape, plus you don't have to tinker with them all the time. They are single coil, but many feel they produce vapor similar to a dual coil. CCV sells them and if you buy a pack of 15 you get a free drip tip. They also work awesome in a tank. CCV sells the Smoktech tanks which are cheap and work very well. I have an Epower which is 3.7V too and similar in size to the Riva and I use the 3.5ml tank. It looks nice and doesn't make your PV really top-heavy like the bigger tanks do on that size PV. CCV also sells pre-punched Ressurectors that have the metal base for use in the Smoktech tanks.
I would def try the Ressurectors and I think you'll really like them. And if you ever want an even more hassle free vape, try one of those 3.5ml tanks and you can go for even longer without refilling. One Epower battery lasts me all day so in the morning I pop a fresh battery in and fill the tank and I can easily go all day and have a maintanace free vape