Your calculations were based on averages, which is ok for getting a rough idea, but doesnt actually tell us what you personally would get.
But tbh, I agree, for cash games the "rakeback" isnt great and that doesnt change on any room that I have seen. But there is some logic behind that.
Say you invest 30-40 minutes playing a single SnG and get $0.50 rake for $5.50 tournament. Thats 10% rake.
Now if you play a cash game for 3-40 minutes, you can easily rake $0.50 in that time. The rake however is half at roughly 5% for most rooms.
So yes, your rake and therefore your rakeback is higher on SnG's, however your rake in terms of BB's per 100 is lower on cash game, making it easier to get a better ROI.
Overall I think the systems are designed to be balanced evenly, but it depends on your play style. If you want to play "break-even" then I would play SnGs as the rakeback is better and bonuses are much easier to clear. However if your focus is win-rate/ROI then I would go for cash game. (sure some people can be exceptions to this rule, but generally it is accurate)
Ok, so in terms of suggestions:
PartyPoker has good traffic, their bonuses arent too hard to clear, but arent going to give you more than 15% rakeback on cash game. They also use Weighted Contributed as their rake model (may have seen the news recently), which means if you play tight, you will make less rake/rakeback.
This is the same for most rooms now tbh and it is a good thing, because it increases the fishiness of the tables, which for people who take the time to learn poker properly, means more profit. So, I would focus less on deposit bonuses as cash game player and focus more on win-rate/ROI.
For this reason I recommend 888/PKR. Both can support 6-tables, fishy as hell, both have deposit bonuses and you should make a killing at the tables. Their cash game traffic is good enough to get a game any time of the day and the fishiness means you dont need to worry too much about table selection.
Check out our reviews for them both below:
PKR
888
So yeah, I would recommend seeing the FDB as just that, a bonus and focusing on rooms that will give your BB's per 100 the biggest boost.
Hope that is useful mate, apologies if the news isnt what you wanted, but unfortunately cash game players will always get less rakeback % because their rake % is lower.