Opus is OK, but I would suggest to use also frameworks, like BMAD. It has really nice agents that can help with brainstorm session, UI/UX, Security, architecture, documentation, and all needed so that the the system has good basis before development.

It is expensive, I know. I mean, it takes a lot of tokens and the $20 is just not enough, but the price worth it, in my opinion. I did it with my project.

I tried, Gemini, GLM, Sonnet, and each of them has it strengths for specific things, but for initial architecture - Opus and BMAd

Actual intelligence, no models are capable of such high level design because they cannot handle the intricacies of the details