Perhaps it makes it a little more likely for a civilization to end themselves but it doesn’t seem to have the potential to be a great filter. It doesn’t seem that likely that even a large scale war with fusion weapons would extinguish a species; and as you point out there is still quite a barrier to development of fusion weapons even with more prolific 235. So far in our history the proliferation of nuclear weapons seems to have discouraged wars of large scope between great powers. In fact two great powers have not fought each other since Japan’s surrender. Granted this is a pretty small sample of time but a race without the ability to rationally choose peace probably has little chance regardless of 235 levels. So if there is a great filter here with species extinguishing themselves in war, more 235 makes it a little bit greater only.
Perhaps it makes it a little more likely for a civilization to end themselves but it doesn’t seem to have the potential to be a great filter. It doesn’t seem that likely that even a large scale war with fusion weapons would extinguish a species; and as you point out there is still quite a barrier to development of fusion weapons even with more prolific 235. So far in our history the proliferation of nuclear weapons seems to have discouraged wars of large scope between great powers. In fact two great powers have not fought each other since Japan’s surrender. Granted this is a pretty small sample of time but a race without the ability to rationally choose peace probably has little chance regardless of 235 levels. So if there is a great filter here with species extinguishing themselves in war, more 235 makes it a little bit greater only.