You can also trade with your own strategies at companies that let you use a mis of your money and theirs, and provide some resources.

I'm not sure I follow you, do you mean leverage or something else?

You could also go to people with established reputations in academic or quantitative finance and show them enough of your method to convince them you're legit, but not so much they can copy it, then have them lend their reputation.

Any information about a method narrows the hypothesis space, so I'm not sure this is possible. But discussion with academics, who presumably are not actually trading is an interesting idea.

I'm not sure to what extent a finance-specialist AI that achieves superhuman performance is really easier to make than a superhuman AGI

Its not at all obvious whether its easier or not - if AI can be broken down into subsystems, such as "planning" "hypothosis generation" and "predictions" then it is a matter of solving just the prediction subsystem, which is of course easier than solving all the other problems as well. OTOH, "hypothosis generation" could be an integral part of prediction, and the subsystems might not be able to operate independently.