I feel that you guys are not understanding well, why casuals were not able to raid at classic in 2019 release.
It is not the leveling speed, it is that no matter how fast you make leveling, that those who have the keys for raid spots, will alwas be faster than those that want to enter the door.
If you make it 40%, the casuals will still need 10 times longer to ding, if you make it 80% it will be 10 times longer, compared to the streamers and their followers.
You can not fix this unless you wait with releasing the raids. Open Classic SOM and let people level up for 2-3 months, then release the first raid. This is the only chance how you may get the result you desire.
We can see this well on era realms, casuals are now raiding there a lot, as the gatekeepers moved to TBC. It is a totally new scenario and fun for those with less time.
With SOM; you will not help casuals if you speed up the leveling process, they will still be miles ahead and once they may be rdy for MC, the majority will sit at AQ and not let them join as they lack experience and gear.
You make the same mistakes again and again, with retail you did boost leveling in such a way that casuals get to 60 in a few days time, but this doesnt change the fact that the gatekeepers and elite players still reach it faster and then demand things that a new level 60 just wont have.
At the same time, you keep making the game unplayable for those not interested in raids at all, but the quests. Remember why you added zones that level with the players in retail? Because people were fed up with grey quests, yet you introduce grey quests now as well at Classic.
From my point of view,
you should offer a SOM realm with premade 60 chars for raiders, who have everything they need to start out with dungeons and then they can hunt for their world first kills.
A second realm should be the untouched Vanilla version, where everyone starts at 0, with slow leveling and community building.
You give both types of players a chance to enjoy their game, instead of making a knee jerk move that doesnt fix any of your problems.