I feel like I've opened up a pandora's box and I don't want to have to try all of them. It's not like they're local, you know? : (
I know exactly what you mean!
Around a year ago, right after I felt I had satisfied my stub usage needs, I got into the world of obliques and everything changed.
I actually don't normally rotate the pen when I write, but I just really like the different angle of line variation you get from using an oblique as opposed to straight nib. Every time I grab one of my obliques, it takes me a few seconds to get back into the groove, but it might be totally natural for you. The reason I got into obliques in the first place is that vintage oblique nibs often have a little more line variation than standard stubbish vintage nibs.
If you prefer stubs then you'll probably be fine getting an oblique stub. It will have the same kind of subtle line variation you are used to. I wanted a little more line variation so I went down the middle road of getting a cursive italic. It is butter smooth, but there is a sweet spot so I have pay attention to my writing posture. A regular italic would be even less forgiving and would take some practice to be able to use for daily writing. I suggest trying a stub and a cursive italic. The cursive italic gives enough line variation that you could use it for some simple calligraphy if you wanted to. If I ever get a chance (i.e., if my wife ever lets me spend the money again haha) to make another order with them, I will definitely order a 1.1 oblique stub. My BB oblique stub is pure juicy goodness! As for rounded nibs smaller than 1.1, since the Knox OB and OBB nibs seem to be decent, I don't think you'd need to order a custom ground OB or OBB.
Unless you naturally turn the pen a lot, 15 degrees will probably be fine. The fpnibs folks have excellent customer service, so you could just email them some pictures of your writing posture from different angles (a video would probably be best) and they can tell you what would be best for you. Contrary to popular belief, writing samples don't really help determine much because they really need to see how you hold the pen to the paper to know how to customize the nib.
I had several emails back and forth with them when I ordered the architect grind because that has to be ground to your exact writing angle in order to feel smooth. I highly recommend them highly enough!