I don't even use completions, really just agent mode. I do planning, wireframing, creating specs all with agents. Even small MVPs created in 5 minutes, deployed in 10, during a meeting to just brainstorm. As for the models. Go with Claude 3.5 or 4.0, GPT5. Use sequentialthinking and Taskmaster MCP. I could write a book about it... but the best way to go about it is to dive into, get frustrated, push through and then learn it the hard way. I started delegating a lot of my programming work the day ChatGPT came out; just copy and pasting, and since that day, my reliance on AI has just been increasing, and I have been getting better at it (and now I am at this stage.. with 2 PMS).
One of the bigger things is when you introduced some bug, start working backwards with the agent, simplifying whatever you build to its bare necessities, and the moment it dissapears, start a new chat, and build it back up to what it was before (in the desired non-bugged state). This often works if you then also switch to a completely different model.
Not OP, but regarding your situation, I suggest moving to an agentic solution instead of “copy-pasting to GPT” — this will boost your coding productivity. There are several tools available, and to each their own, but try out Claude Code.
Are you using agentic features, given that you have not just one but two PMs?