As an outsider who's been though this before, I'll give you my take:
1. You gotta look at the coaches. Can you upgrade from Bud and get someone who can get more out of these guys than him. If the answer is unclear, you gotta look at the roster. Middleton has shown that he's a very good regular season player but more of a Tobias Harris type than you know, a legit #2. With his contract, you know any trade back will get you 40-45 cents on the dollar so he's not worth moving unless you are just building fresh. Next is Bledsoe who's on a reasonable contract for his impact. You could probably get something sound for him if packaged with some other assets. The question is what could you get? Not much honestly. He plays the deepest position in the NBA and teams are dying for a PG who's more of a combo and cannot create for others. These types like Eric and Dennis Schroder have value but not much.
2. What's my trade options if I don't have good assets. You got to look at the rough assets and see who can help? Can Horford help? Can Tobias Harris help? Can Blake Griffin help? Can Chris Paul help? Can Andre Drummond help? CP3 clearly can help but sadly, you will have competition with Philly who has more assets and more willing to overpay due to having their squad for at least two more seasons without question. If you miss on CP3, who on this market could help? I am not sure and at the cost of Bledsoe might also just defeat the purpose as well. It's tough but Horst has his work cut out for him.
3. You can look at the bubble as unusual and make another run at it regardless with or without Bud. Why not? Best regular-season team. Get homecourt advantage and maybe you get lucky with the seeding. Avoid Boston, Miami, and other teams and maybe get Washington and teams like that instead. Sadly, the biggest mistake I see from MIL is some of the contracts like Middleton. A max cat has to be much better than this. Him and Tobias Harris has been big letdowns when it comes to pay to impact play. Giannis can't do it by himself.
4. Considering trading Giannis. He's a rental so generally, you will get 25 cents on a dollar unless it's to a team he wants to go to. So ask Giannis, if traded, where do you want to go. He says, Golden State, Miami, or Toronto. You look at GS, they have #2 overall, a top 5 protected pick next year from MIN, and Andrew Wiggins bad contract with limited impact. Does that appeal to Horst, IDK. It's highly unlikely they trade Green, Klay, or Steph.
Then you got Miami, who already made it clear they aren't trading Herro, Bam, or Butler. They do have guys like Robinson, and some others that could be appealing. Toronto has a number of guys like Powell, but no Siakam, O.G. or Lowry. Sadly, you need an idiot team like the Lakers who are desperate with a superstar already. Without one, my guess is Denver could have the best offer with MPJ, Gary Harris, and filler for a rental. That said, there is probably zero chance that Denver will do this deal without assurances that Giannis will resign so even then, it's unlikely. It's tough, teams aren't going to trade you major assets for a player who can Kawhi them especially if you can't win it all with them either.
I can see Boston considering something like #14/26, Hayward, and future assets for Giannis but I am not sure that's all that appealing for MIL. Maybe the best deals could come with a Giannis/Middleton package deal. That could maybe get two quality young assets. Might be something to consider.
In my opinion, run it back and if Giannis leaves, he leaves is the best option. But I can understand if Horst trades him, starts the youth movement, and hits reset. This is why, it's best to keep Bud until you know what's your trade options because you don't want to fire Bud if you are trading Giannis but if you keep him, you have to really think about it.