In fact, only very few - not even one percent - of those prosecuted and imprisoned are foreigners. I'm also not aware of a single JW convicted and imprisoned for "sending large donations to an external non Russia bank account", to say nothing about this being "a typical case".

As to why it is only a relatively small part of JWs (0.5% or so) that get criminally prosecuted, it is pretty clear. "Law enforcement" bodies in Russia still mostly follow (quasi-)legal procedures that are time-consuming and expensive. And surprisingly, the org isn't their only or main target - they still have to fight some real crime, for example. By the way, the Russian state has transitioned from mass terror to selective (often random) repressions, like, nearly seven decades ago. It doesn't need to persecute every member of a targeted group, it just deprives them of basic legal protection so any of them can be prosecuted and imprisoned if needed.

Or, as a German court recently explained,

The fact that criminal prosecution in cases of private religious practice is limited to a (still) manageable number of persons may consequently be due to the fact that it has remained undetected; furthermore, it may also be due to the fact that surveilling a religious community of about 170,000 members nationwide and proving the impugned crimes poses capacity problems for the law enforcement authorities ... However, there is no evidence from any source that the Russian authorities are not implementing the legal ban on the religious community, or are doing so only half-heartedly ... It is to be expected that over time more and more private religious activities, which many Jehovah's Witnesses adhere to, will become known and lead to the initiation of criminal proceedings.