How long after stopping a canister before cycles stop decreasing?

Some maybe. But certainly not all you suggested:

  • cycles usage web dashboard: Anything that’s web-something will have to be an asset canister.
  • specific purpose database: that’s a userland feature. Take a look at e.g. Sudograph or CanDB
  • real value asset platform level system: what is that meant to be? The ICP Ledger is actually NOT a part of the platform. It’s implemented as a normal canister.

Not really, but if you have a nice contribution to make there’s always a way to do it. Just reach out in case you have something.