The Far Realm seems like it's supposed to be a place of horror, which doesn't come across in these monsters very well. I think most of that is the presentation "The Far Realm is super creepy... here are the tactical considerations of fighting things it's touched."
If they're going to focus on the fight, which I totally understand, they should really push the horror flavor to the forefront in the fight itself.
For example, if foulspawn are attached to a gate, give us some mechanics for the gate. Maybe taking long rests near the gate moves you one step closer to being a foulspawn, which can only be reversed after closing the gate. Certain foulspawn could advance the condition on exposure.
Or play up the absorption horror for the mouther. Maybe it can absorb a victim easily, but the victim remains entirely aware and can make Will saves to take an action as the mouther. But, until you're freed, each round also advances the looming risk of insanity and physical assimilation.
The "this thing is creepy, make a save or be creeped out" is a pretty weak way to represent horror. Even without a mechanical basis, some more focus on their horrific origins would go a long way.
Cheers!
Kinak