Current state of my workflow: (your feedback is welcome, I'm an autodidact and have been self-teaching myself thus far) I also think my workflow addresses some of the pain points the OP mentioned.

If a problem is a continuation of the current or other chat, switch to it. If it is a new problem or sub-problem requiring something more extensive than a tiny refactor, a new chat is started.

From there,

Start in Ask mode. Ask about existing code I'm trying to modify. If I am interfacing with someone else's code, that is put in reach of the project and ask questions about how it produces certain results or what a function does. Ask the foundational 'bottom-up' questions, how does this work? what routine produces x? Call out specific external sources from the web if it contains relevant information, like an API. Iterate until I feel I have a grasp of what I can build with. Not only does this help me comprehend the boundaries in terms of existing capability and/or shortcomings, it seeds the context.

Move to Plan mode. Provide a robust problem statement and incorporate findings from the Ask into the problem statement, and what the desired output is. Throw in some guard rails to narrow the search path used by the LLM as it seeks the solution. Disqualify certain approaches if necessary. If the LLMs plan isn't aligned with my goals, or I remember that thing I skipped, I amend the plan. The plan prompt I typed is saved to a blank file in the text editor.

Implement.

Validate. If it works, great. Read the code and approve each change, usually I speed read this.

If it doesn't work, I tell the LLM the difference between the expected and actual result and instruct it to instrument the code to produce trace output. Then feed the trace output back into it with explanations of where the output doesn't match my expectations (often times revealing weaknesses in my problem statement). Sometimes when it is a corner case that is problematic, several iterations are required then screen for regressions. If I reach the point where I know I screwed up the planning prompt, I trash the changeds, then I revise the copypasta saved earlier and start a new Planning session.