setValueNotifyingHost is the correct function to notify the host about a changed parameter value. There are no suitable alternatives as far as I’m aware.
Do you know for a fact that the desired behaviour is possible in REAPER? Are you aware of any VST(3) plugins that are able to update REAPER’s automation envelope when their UIs are closed? Unless other plugins are successfully doing this, I’d assume that this is an issue in REAPER rather than in JUCE.
There’s another similar issue report here: Reaper does not write param automation if GUI is closed
Perhaps it’s worth asking on the REAPER forum whether the behaviour you’re seeing is intentional (and maybe linking back to these threads too).