Just to make sure I understand, with this approach, we are basically just categorizing the Schwab linked account transactions, and not much more than that.

I have been keeping the books manually in Excel for many years (as it may be obvious).  Having QB linked accounts is great, albeit a huge learning curve.  Your recommendation not to use sub-accounts presents a few problems I cannot figure out, which is that (1) securities purchased from the Schwab account show up as Expenses in reports such as Statement of Financial Position, and (2) money markets sold show up as Profit, even though it's just the principle.  

All interest earned shows up as a separate Received transaction that I can categorize as nonprofit revenue/interest income.