these two instances' open registration policy, which is extremely problematic for us given how federation works and how trivial it makes trolling, harassment, and other undesirable behavior;
Doesn't this just lead you down a path of simply defederating most instances? It sounds like you just don't actually want to participate in the larger federated community. Which is admittedly fine if that's the direction you want to take this... but it certainly seems like a first step towards just running a closed-off site with none of the actual benefits of being part of the fediverse.
This is hilarious, they know exactly how to cater to the annoyed discord userbase.