as others have said, I think the Bucks and Sixers are a cut above the rest, but they're not so overwhelmingly talented that I can imagine some other team making it to the Finals.
The Bucks are nothing without Giannis, and their supporting cast is thin enough now that if Bledsoe or Middleton has an injury they may be sunk.
The Sixers could be in trouble in crunch time due to their lack of ballhandlers, though adding Horford means they are much less vulnerable to getting wiped out if Embiid has to miss a couple games.
Yeah, to answer the question, "is the East wide open", I'd say absolutely.
MIL and PHI will be the favorites no doubt out of the East, but as you said, I don't think they are "overwhelming favorites" over some of the other teams right behind them.
In-season trades, signings and/or injuries will also be a factor, plus the impending Westbrook trade likely to MIA/DET which makes the East very interesting.
Horford was a phenomenal get for PHI, but I kind of question their shooting now that Butler + Reddick are gone and assuming Simmons still doesn't know how to shoot. Likewise, every team (including us) has a lot of question marks in certain areas.