you only need to be "on" to make transactions. In the case of sending transactions, you only need to broadcast the transaction itself (500 bytes), with no further traffic needed. The blockchain data is also broadcast via a satellite: https://www.blockstream.com/satellite/
>transferring non-trivial amounts of data
SPV clients require very little traffic, on the order of tens of kilobytes. Full nodes only need to download under 2MB worth every 10 minutes, or 3.33 KB/s. That's very easy to sneak under the radar if need be.
>Do you really think it's that hard to block port 8333 and a list of known hosts
good thing you can run bitcoin as a tor hidden service.
>If this is your threat model, cash is much safer.
Except you need to be in-person to make transactions.
For a repressive government, that means that most people will steer clear which makes it easier to filter since you don't have a large volume of innocent traffic complicating analysis and by creating a great mechanism to get people to install spyware by circulating news underground of a great client which is preconfigured to evade the filter.
If this is your threat model, cash is much safer.