Most people don't remember that email used to work offline. You would go online to send your outbox and receive to your inbox and after five minutes or whatever it takes to sync thee immense 5kB you cut this expensive line again. Then you can read through stuff and reply offline.
Yes, nostr should at least optionally work like that and it makes a lot of sense to split the relay (store and forward) part from the client. A client shouldn't have to care about storage or networking for that matter.
I find myself going back to your NostrPost and seeing if the relay implementation over there can run as an Android app. We just need a database.
NostrPostr was my rookie attempt at doing this while getting to know nostr and I gave up when I ran into some concurrency issues. It worked locally when testing but not when it got hammered by multiple clients. But for a local process on your phone, maybe that's not even an immediate issue. Fingers crossed.
Think and do offline, broadcast online or find a generally offline system (like radio / non-internet method to broadcast)
nostr:nevent1qqsycf36vtk5yulwecr0xh7m7z9seflfhytyed4g0k67uxurayg7s6qprpmhxue69uhhyetvv9ujuumwdae8gtnnda3kjctvqyxhwumn8ghj7mn0wvhxcmmvqywhwumn8ghj7mn0wd68yttsw43zuam9d3kx7unyv4ezumn9wsq3camnwvaz7tmwdaehgu3wd46hg6tw09mkzmrvv46zucm0d5q3gamnwvaz7tmjv4kxz7fwv3sk6atn9e5k7qg5waehxw309ahx7um5wghx77r5wghxgetkqyw8wumn8ghj7mn0wd68ytnzd96xxmmfdejhytnnda3kjctvqyv8wumn8ghj7un9d3shjtn5dashxarj9eehqctrv5q3vamnwvaz7tmjv4kxz7fwwpexjmtpdshxuet5qy28wumn8ghj7un9d3shjtn90p5hgtnsw43qrvflhc
I think this was less to do with size of data and more to do with constraints of the era.. Pay per Time connected/Limited time units per month, and the need to use the phone line for voice.