[neilv]

For a non-startup personal open source project, you can consolidate all that data in the desktop program https://gnucash.org/ .

Offhand, I suspect you could layer all those features you mention, using a mix of GnuCash customizable existing reports, and their (poorly documented) extension in Guile.

The UX might not be what you want with just reports and scripting, but you can definitely analyze data and generate arbitrary noninteractive HTML reports, and there's some limited facility for dialogs that way. (For example, the features to help keep an ITOT & AGG ETF portfolio balanced for risk tolerance seems trivial, and it could even have a sense of calendar schedule for periodic rebalancing. But instead of it doing the rebalancing for you, the UX would probably just be a generated report that shows the balance, and tells you the exact two transactions to rebalance.)

Or, alternatively, get into the code of GnuCash itself, and do whatever interactive GUI and maybe even (if very brave/foolish) making your code talk to your brokerage, etc.