Not sure about creating posts but blorp can at least cache posts for offline viewing.
Free and Open Source Lemmy Apps
A home for discussion of Free and Open Source Lemmy apps and tools for all platforms.
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Blorp in theory could do this. It would need a bit of tuning to really work well under the circumstances you mentioned. For example, feeds only refresh currently when you open them, but I could have it auto refresh all your subscribed feeds when you connect to the internet.
Draft posts and multi account support is already there if you wanna give that part a try. Writing comments offline wont work yet. They may save if you don't close the app before reconnecting to wifi, but I wouldn't count on that.
I'm the developer behind Blorp btw.
Thanks for the tip. It needs AOS 8, which blows it for me using my phone. Though it could motivate me to hack AOS onto a Kobo e-reader just to run Blorp.