Based on the information provided in the knowledge sources, it appears that PowerShell (pwsh) should indeed be available as an option in Warp's startup shell settings after installation. However, your experience suggests there might be an issue or a discrepancy between the documentation and the current implementation.
According to the Warp documentation (https://docs.warp.dev/getting-started/using-warp-with-shells#using-powershell-pwsh-with-warp-on-macos), PowerShell is supported as a startup shell option. The documentation states that you can set pwsh as the default shell for Warp in the session settings (Settings > Features > Session).
However, it's worth noting that PowerShell support in Warp is a relatively recent addition. The Warp changelog (https://docs.warp.dev/getting-started/changelog#id-2024.09.26-v0.2024.09.24.08.02) indicates that PowerShell support was officially added in version v0.2024.09.24.08.02, released on 2024.09.26. If you're using an older version of Warp, this could explain why PowerShell is not appearing as an option.
Your experience of having to manually add PowerShell as a custom option suggests that there might be an issue with Warp detecting your PowerShell installation. This could be due to various factors, such as the specific version of Warp you're using or how PowerShell was installed.
Regarding your observation about launching PowerShell by typing pwsh in zsh not providing the same experience as opening it in a new session, this is expected behavior. The Warp GitHub issue (https://github.com/warpdotdev/Warp/issues/4199) mentions that "pwsh warpification isn't supported at this time. You can still run pwsh, but Warp features (i.e. Blocks, autocomplete, etc.) won't work within that session and it'll run similar to in a normal terminal."