OP's startup idea is in a well-understood domain. There are lots of existing competitors offering basically that exact service. There are clear, obvious paths to building it. GPT can draw from blog posts and tutorials.
On the other hand, it does poorly with significantly novel ideas. I tried it with one I've been working on that combines 3D printing and crafting in new ways. Its answers just regurgitated blog posts about existing combinations. It even ignored the grammar of my questions, which switched the usual direct object with the object of the preposition. Which makes sense, because it isn't doing critical thinking - it's pattern matching.
And coding is supposedly an area where it shines.
GPT is waaay better than us humans at finding and summarizing information. Way way faster. But I've tried to write a blog post about some technical stuff and I finally gave up after many tries.
It's good. It's frightening already. But it's still not "there". It's like having a extremely fast but bit dumb virtual assistant.
Way better than anything we had before.
Is that true, though? These AIs seem pretty capable to extract abstractions, like an author's style, and apply it to novel concepts.
Also consider how long it takes you to bake a feature into code you have never seen before to code that you wrote.
If you ask it about AWS - you need to already know AWS, same with business etc.
And if you can't make your own elevator pitch - the odds that you will be able to run a successful company are little to none.
Website builders and ready-to-use templates have been around for decades and they only increased the demand for devs.
Even if GPT will be able to produce complete system from start to end - those systems will need support, fixes, etc. Sure you will need less engineers per system but the amount of systems will increase.