So the issue with Jaylen being a part of the Giannis deal is that:
I'm not 100% sure Jaylen IS more valuable than the heat package, depending on what the Heat package is. On draft nice the Heat could in theory trade #13, 29', 31, 33' first rounders along with swaps in 28', 30', 32' plus Hero, Jaku, Jaquez and Ware*. Now obviously we don't know if all of that is on the table, but if I'm the Bucks I might prefer the pick heavy package from a team that will be locked into Giannis/Bam at Giannis' age 33-38 seasons over Jaylen.
Now, it depends on what you can spin Jaylen off for. If you can get back your own picks from Portland then that exceeds any MIA offer, but it's not obvious Portland would do that. I can see a scenario where they'd rather just keep those picks with the Bucks imploding. There's no other pick-based offer out there that is so obviously better than a bunch of Mia future draft capital. It would be a bit of a bet, but not a bad one.
Which leads to the whole problem with a Jaylen based offer: He'd the best individual piece the Bucks could get, but he's not the KIND of piece they want. Sure, Boston could blow away the MIA offer by doing Jaylen and a bunch of their own picks but they you get into the conversation of why you'd trade BOTH JAYLEN AND ALL YOUR PICKS. That's too much.
This is why 3 team teams are hard. You need to find a team that wants Jaylen at a 35% max, has the assets to offer a similar price to what Giannis would go for, and wouldn't simply pay that price for Giannis directly.