What's the best way to implement long delays (days, months) with persisting messages between reboots
The cron-plus node can be dynamically configured via a msg input and has persistence built in.
Look at the built in examples (ctrl+I --> examples) and the built in documentation (on the side bar)
And if you need to persist some data, that can be done with the filesystem persistence available to context/flow/global variables. However, note that these - I think - are retained in memory as well. So you might want to consider external storage for larger amounts of data. A simple JSON file may be enough or possibly a database depending on the size and complexity of the data.