Jaysus, that's a lot of individual things to go through. I'll try to address them individually and then general comments afterwards.
One thing I will point out right now though: It seems like your expectations might be a bit high. You're asking for conflicting things. Like chain vaping all day, wanting a warm hit, but not wanting to burn through e-liquid.
I plan to buy the OBS Engine and the RBA for the Smok big beast.
I would get the OBS Engine
Nano, and keep buying coils for the Smok tank. The tank is designed for coils, the RBA deck is an after thought. It's like taking your elderly neighbor's station wagon and wanting to take it street racing.
If you want to get a big dual coil RTA get a big dual coil RTA. Don't get a family sedan and try to slap giant wing on it.
I like a slightly warm vape. Not hot but a little warm.
We'll keep this in mind. Aim towards vaping between 4.5-4.7V. With a single coil RTA like the OBS Engine Nano, you can make a robust coil that can take 5V and vape great.
I don't work and if I am awake I am chain vaping.
I work from home so I am chain vaping 15-20ml a day, sometimes more. It's no problem with an RTA.
I also would like them to last awhile between changes.
You can get a coil to last upwards of a month, pushing half a litre of e-liquid through it. But you'll need to change your wicking periodically, every couple days. And clean the coil by dry burning it.
I have sworn off dark liquids for now. I'm all about flavor.
Dark liquids, super sweet liquids; doesn't even need to be a consideration anymore. Just change your wicks when they need to be changed. I've vaped things that look like coffee.
I am going to buy from Advanced Vape supply. I don't want to build my own and don't mind buying them prebuilt.
Looks like they only sell stainless steel. I don't TC so I can't give advice regarding stainless. I'll continue and give advice from an NI80/Kanthal point of view. Keep this in mind, there may be some points that differ.
I have never used a Clapton but gather that I will want some sort of Clapton.
The point of a clapton is that it efficiently packs in surface area. It means you can make reasonable sized builds while having 2-5x the surface area. It also means you burn through juice faster.
I don't want to go threw 5 mls of liquid every 2 hours.
Chain vaping all day you will be consuming juice. There's no magic around it.
So something that can keep up if I have enough cotton or not too much.
It's important to learn to wick properly. If you do, you can chain vape all day and not worry about dry hits. (Assuming you have a good tank, like the OBS Nano).
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I'm going to have to ignore the juice consumption concern. You're asking for too many things in one solution.
First thing, get a single coil RTA. It makes building easier. I don't mean that in the sense that there's less wire to coil or mount, I mean that in the sense that it's easier to hit a reasonable resistance.
Second thing, I would suggest you get something like a 26g core with a 36g wrap - it's pretty standard. You can use double or triple cores if you want, but a single 26g core will be a good starting spot.
Here's a build with a 6 wrap @ 3mm id. I think it's a great starting point for a beginner, coming in at 0.5 ohms. I've also included the calculation for 4.7V (warm vape), meaning you should fire it around 43W.