First, No bombers are needed to fight ISIS. Each military equipment, is built to achieve some goals. CAS, or air to air combat , or some other goals.. you need to always keep it mind.
Bombers are useless in this case, since ISIS is a partisan group, and not a standing army. They don't have any important buildings, or strategic factories, or ... either. So, bombers, or strategic bombers are totally useless. What you are saying is using strategic bombers to provide CAS, while in fact, providing CAS against ISIS does not need and cannot be well provided by strategic bombers in this case since ISIS is not a classic army. So, even this huge payload of bombs is also useless. In this case, you need to have an attacker with a big gun, a good loiter, which gets close to ground, and pilot can recognize ISIS forces. ISIS is a very fast moving military group which sometimes shelters among civilian houses, or ... there are many technical difficulties in this case. so, pilot needs to get close to ground to clearly see everything, otherwise, you would kill bunch of civilians instead of ISIS. Now, by this job description, we can see that A-10 is perfect for this job, since it gets close to ground, has an amazing big gun, very good loiter and is very strong against ground fires of ISIS partisans. Iran does not have A-10, so the next best option would be Su-25.