When you say a “better” bike, what new components do you think will make you go faster, given that you have already upgraded your wheels?
You want a quality alu frame, but with respect, you already have one 🙂
So you already have:
1. A good frame, in your preferred material
2. The right finishing kit for you
3. Decent wheels
It sounds like if you just upgraded your groupset, you’d get the bike you want!
Of course, you don’t have to rationalise a new bike purchase. If you want one, just get one, but don’t pin your hopes on a new machine, very similar to you old machine, making you much faster!
As for building your own, I think you’ll find that it would be more expensive, even after adding your own bars/stem/saddle etc. Given that you will already have at least some of those parts, I personally would be inclined to buy a complete bike and either specify your ideal stem length etc in the original purchase (a decent dealer will do this for you) or just swap out for your own when you get the bike home.
Of course, once again, building a bike is fun and interesting but if you’re using new parts, will usually work out more expensive than an off-the-shelf bike.
Kudos to you for racing, though. I don’t have the bottle for it. Or the legs 🙂