It's a good idea, and yes something should be done to make this tournament a bit more fair / interesting.
But the B3 format works because among 110 to 120 teams, 32 of them reach the final phase. Which means you can be qualified with a 6-2 record. And each season, 2 to 8 teams finish the round-robin stage undefeated, representing an average of 4%.
The biggest BB countries (Spain and Italia) have more than 1100 active teams. So on average 50 teams would be undefeated after the first 8 games. And your proposal is that only 8 teams can qualify to the next round (quarterfinals), which means an undefeated team would only have a 16% probability of qualifying.
And it's about the same in medium-sized countries like yours. As soon as you get one defeat, chances are you won't make it to the next phase. So what's the point of having a group or round-robin phase if any defeat means you're eliminated ?
All of that without mentionning that a round-robin phase makes sense with teams that are around the same level, while the draw can be extremely unfair when it mixes teams from 50k$ to 1M+ salaries. And to me the percentage of undefeated teams after 8 games would be even higher in this configuration, cause that's the global density which makes an 8-0 record really hard to get in the B3. So the 4% approximation I used should probably be increased.