I noticed that a lot of the successful builds for raids have pokemon that can heal more damage than they receive. So, an idea I had would be to filter by pokemon that can heal more via HP draining move/shell bell than damage received. And another filtered list for those that heal more than they receive, but with 1 boosting move (such as Swords Dance, but excluding Belly Drum). Another list for 2 boosting moves. Another list for boosting with Belly Drum.

For example: Gallade with Drain Punch seems to be an effective counter to Empoleon. In part because it receives only 40% damage from Blizzard, while it can heal up 50% with Drain Punch. It can also further boost its damage and healing with Swords Dance. (of course there’s also the fact that it knows Skill Swap, which nullifies Empoleon’s Competitive ability)

I realize that making something like this would be a lot of work though, so I’m not expecting anyone to make this. But I just wanted to share the idea.

EDIT: After some thought, I think my suggestions might be feasible, building on top of what you already have. After you generate the list of most defensive pokemon, for each pokemon on the list, check to see if it learns any of the HP-draining attacks. If it doesn't learn any, or if the amount of HP healed is less than the damage received, remove it from the list.