Honestly, use Pi, start the session with K3, design it, store the design + chat history + todo with phases + why + readme then when you are at the time to build do `/model` select deepseek and let it rip!!

You can even make a Pi extension to let it switch models on its own, then tell it to switch to one or the other for a given task. It's not perfect, but it is pretty reliable once you have the instructions dialed in.

Here's mine: https://pi.dev/packages/@arcanemachine/pi-model-switcher

I have mine configured to use a "thinker" model for planning and decision making, and a "doer" model for the easy stuff (digging around the repo, writing code based on the thinker's instructions, etc.).

I run a similar setup, just with different models. I also use herdr, which lets me have the same project worked on side by side by different models, each in its own git worktree.

https://github.com/earendil-works/pi

https://github.com/herdrdev/herdr