I like the simplicity of things you made. It's all straight forward.
Only missing things:
On 3rd step of proposal there is no way to use products/services/subscriptions which are saved in catalog making catalog useless.
It will be also useful to add a option to "save to catalog".
On same page in dropdown for taxes add option to add tax (going on tax page before creating a proposal is not intuitive for new user).
Good job!