it isnt the rta, it is the actual 30w mod you are using. It just isn't powerful enough to power more than a single coil, and only around 26g min. You could go with a dual coil 32ga but that wouldn't give you much satisfaction. 3.0 core should be fine, even with 6 wraps. I allowed a little bit of room for adjustment. 5 wraps on a 3.0 id core might be a bit better.
Now I will say, you can build some pretty impressive single coil builds. For example, I am using a 3 strand, 7 wrap, 32ga stainless steel coil on my kanger toptank mini rba., 5 wraps would probably be about right for 30w, but.. that is like 15 loops so still a really decent vape.
I think, it is better to run 1 good single coil build, that a bad dual coil, right?
You can use that link I sent earlier to play around with your coil before actually building it. select your wire, and core size, change the desired ohms, and you want your heat flux above say.. 300. 350 would be even better.
The suntan looks pretty good. But same problem.. You only have 30w to work with. BUT... you can work with that 30w and get a decent vape. The juice holes are large and lots of airflow.