Here are my thoughts on your questions. As usual; these are
only my opinions, and may be based solely on my own experiences. Others may have an entirely different point of view.
1) So I’ve found, through flavor testing, that a great tasting flavor in my RDA is too weak in a sub-ohm tank. What is the standard approach here?
I have found that just about every different atomizer will cause the same e-liquid to taste differently. Different coils, and/or different wicking, can also influence flavor.
Ideally; I want to do all my testing in the same atomizer I intend to vape it in. When that isn't convenient/practical; I attempt to mimic that setup as much as I can. I use the same coil(s) and wicking material. I try to get the airflow, and power/temperature settings, as similar as my "everyday" device as I can.
I find; the greater the difference my "testing" setup is, compared to my "routine use" setup, the less helpful (informative?) it becomes. The testing feedback is less "true."
I also try to never combine my hardware tests with my DIY tests. I try to always test my DIY with my "everyday" setups. I try to always test hardware with e-liquids I am very familiar/comfortable with.
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2) Although I don’t have a desire to try tobacco flavors, is it sometimes necessary to use in low percentages to add that something to a recipe you can’t get otherwise? I’ve been told this and the example used was TFA RY4 Double in some bakery recipes. I know taste is subjective but I’m curious of the consensus concerning this topic.
Tobaccos can be a strange class of flavors. As a new vaper; I expected them to taste the way cigarettes tasted when I smoked. I found them to be nothing like that; and thought they were gross. And, for some odd reason, they were the one flavor profile that made me jones for a real cigarette. I steered clear for more than three years. Less that two years ago I revisited tobaccos and began enjoying them.
I share a similar opinion of RY4's in general, and TFA RY4D in particular, that they/it is about as "non-tobacco" as a tobacco can be. It is mostly caramel, with a little vanilla, and some sweet. I have never actually been able to pick out an actual tobacco note.
I don't ever poo-poo
any flavor combinations anymore. As I have journeyed on my own DIY adventure; I have learned there are some surprising, and useful, flavor pairings. Often, the secondary/supporting flavor is not even noticeable/recognizable to me. But, its presence is missed if it is not included.
Like most DIY "tips," all I can suggest is to try it, and see if it works for
you.
3) I’m using stainless steel clapton in my RDA for flavor testing. I noticed in a couple of milder flavors and in additives I taste a metal / metallic taste. Is this normal? Is there another metal that would work better for truer flavor testing without adding flavor not in the sample? (I’m familiar with Ohm’s law and different metals and settings. So no worries there. However, I’m even newer with RDA’s than I am to DIY ejuice

)
I have found, that all new coils, have a "break in" period. I can never be sure if it is the coil, the wick, or a combination of both. All I know is I get an "off" taste for about the first 5 minutes of vaping on every fresh build.
If you want to get rid of it, and don't want to waste your testing mix; vape some plain unflavored e-liquid until it is no longer noticeable.
Also, if you have not used SS coils before, you might get some useful info here:
Stainless Steel Coil & Wire Instructions and Tips