I've been watching a few videos and in all honesty, I've barely seen any fighting and the way they've fairly easily moved into surrounding suburbs raises my suspicions. I guess ISIS found no point in extending themselves or revealing themselves to coalition air-strikes and would rather bunker down in central Raqqa. In that case, I would imagine it would take months to see any significant gains without taking huge losses. Although, using the strategy you mention, it could most definitely reduce the effort and time needed to liberate the city.
There's been a strong increase on attacks Deir Ez-Zor so I'm wondering if these are re-inforcements from Raqqa or more of a desperation attempt from the remaining fighters there to break the resistance. I can't see how the Russians would let anyone escape to Deir Ez-Ezor because if the SAA and its allies lose that area it would signal a huge shift in the geopolitical landscape.
Just my thoughts, but you've been following the conflict a lot more than me so I could be off. Cheers.