I really love both of these players, but I think that comparing them to the Braves pre-arb extensions is a bit of Fool's Gold. If you look at the players they gave pre-arb extensions to (Albies, Acuna, Strider, Bummer and Harris), I would argue that, outside of Bummer (whose extension is $16M total), they are all more "complete" players at this point than either Bello or Casas. Also, Albies contract is just plain ridiculous (and has been discussed here ad nauseum).
Albies is a 4+ WAR player, Acuna is an MVP, Strider is a Cy Young candidate, and Harris is a nasty combination of offense and defense. All of them are what I would call 4+ tool players (except Acuna, who is like a 10 tool player).
Riley wasn't signed until he was in arbitration already, and Olson and Murphy came over in a trade before signing their extensions.
Neither Bello nor Casas has come close to the production of these players. I am pretty convinced that Casas is going to be a monster offensive force this year, and if I had to pick one of them to extend it would be him, but even he has his warts and I would call him a 3+ tool player at this point. Bello is a ton of fun to dream on, but he is essentially the same age as Strider and has barely half the K%, a worse HR%, and is lagging almost 30 points behind in ERA+.
Again, I want to be very clear, I am a big, big fan of both of these players. I would love to see both sign to extensions and spend many years with the Red Sox, but I also don't think that it is "necessary" at this point, for lack of being able to come up with a better term.