Siakam+ for Giannis is a no brainer. Even if Giannis leaves after a year like Kawhi did, it would at least give the Raptors another legit shot at a championship, something they don't have as currently constructed and likely will not have if they keep this existing group together (Pascal/Fred/OG). The latter is a very good regular season team but likely a 2nd round exit most years. The 2018-19 season should clearly illustrate that when a top 3-5 player becomes available and you have the assets to get him, then you do it even if it's a rental. Pascal is a better all around player than DeRozan, but impact-wise, I don't think he's much different as a #1 option. As a 2nd/3rd option on a championship contender, he'd be great (as he proved in 2018-19). So the Raptors need to either acquire a player better than him while holding on to him at the same time, or move him for a piece that would put the team in a better position to contend for a title.
However, I don't think the Bucks trade him to the East, if they trade him at all. More likely they trade him West, although PG and AD were traded within the conference, so who knows. Bucks wouldn't have a ton of leverage if they moved him since teams know he only has a year left. If I were the Bucks, I'd just keep him for a year and try everything to win a title next season. If it fails, it fails, but at least in that scenario they would have exhausted all their chances with him.