The current default “Homepage” value is “All”. That makes sense for users with zero subscribed magazines, so they see something.
But for users with one or more subscribed magazines, the default value for “Homepage” needs to be “Subscriptions”.
That’s partially just because subscriptions mean little if the default frontpage ignores them. But there’s a more important reason than that.
The current situation is that posts in m/Conservative are heavily downvoted, and barely upvoted. (Whatever your politics may be, consider if this was happening to you.) As more ex-redditors join, this problem is likely to worsen.
Eventually moderators need better controls, to say things like “Only members can vote”, etc.
But for now, a very easy fix is to change the default value of “Homepage” to be “Subscriptions”. Of course, “All” should still be a selectable option for those who prefer it.
Brigading is a coordinated effort that starts in one or more external communities and is directed at a specific target. That’s not what you’re describing.
Perhaps it’s the case that a Conservative community is just unpopular among users browsing /all? This isn’t a strictly USA politics thing. You’re getting international users seeing this content too.
Even with your proposed change - which I think I agree with, but not for the same reason - nothing is going to prevent users from browsing and potentially downvoting submissions.
If there is this much sensitivity to meaningless Web points (seriously, there are no karma totals here), then I think a better solution might be to find or start an instance run by people who share Conservative values and have that instance control the desired level of federation with other instances.