If you hold these assumptions robustly, the most direct answer would be to focus on the kind of beings who are likely to experience greater suffering by default, namely factory-farmed animals, and potentially some wild animals. You should focus on interventions (alternative proteins, vegan advocacy) that are likely to cause these animals not to come into existence, rather than welfarist approaches that improve the lives of animals but keep numbers relatively constant. This is a very popular approach, so you'd be welcome in this part of the EA space.

But you might want to slightly relax your assumptions slightly when considering practical work you could do. Assuming that reducing suffering is your ultimate goal, if the "best way not to suffer is not to live", it doesn't necessarily follow that the most effective way to reduce suffering (given limited resources) is stopping beings coming into existence. 

For example, an intervention to help people in poor countries detect particularly painful congenital defects before birth and terminate these pregnancies might reduce suffering and satisfy your assumptions, but if it's expensive, it might be more effective to reduce the suffering of existing people, for example, providing relatively cheap pain relief for people with late-stage cancer. 

Or if you could cause x number of factory-farmed chickens to be raised in a free-range/ organic way for the same cost/ resources as stopping y number of factory-farmed chickens being born, there's probably some number for x and y for which you'd choose the first option.