Migrating Chats between iOS and Android is not very easy, this applies to
Threema as well as for WhatsApp. For the latter one you can use the
WazzapMigrator
app/tool, but for Threema there does not exist an "convinient" migration
app yet.
Hence, I wrote a little python tool, which makes the
migration for you a bit easier. However, you still have
to do a lot of "manual work", as described in the following steps.
For now, check out the GitHub page for more details.