Migrations take time, it was never going to be a flick of a switch. Reddit content will slowly get worse and worse, and Lemmy content (or some other competitor) will get better and better as more people move. it’s those core users that generate great discussion that matter the most and those people are looking for somewhere else.
depends on how many services you plan on running, i think the i3 would be sufficient for what you listed, the i5 would give you room to grow.
the i7’s usually aren’t worth it for servers since they are just a clock speed increase.