The exception is significant because you assumed it to be significant ("the Suns could very well make a huge offer ...").
Unless I were really in love with Jarrett Culver (after seeing his tournament, I'm not) or DeAndre Hunter (after seeing his birth certificate, I'm not), I wouldn't make the deal you proposed. The Bucks 2020 pick is basically a second-rounder. The Suns pick would be more promising, but it's very possible that adding Ja to Booker, Ayton, Josh Jackson, etc. will finally make that team go.
Let's put some numbers on it. Would you trade the #3 overall pick for the #4 overall pick this year and the #12 and #30 overall picks next year? Especially knowing who is going to be available at #3 and #4?
Again, if you really like Culver or Hunter or Cam Reddish (gack) or Bol-Squared or whoever, then maybe you make that deal. I see a significant drop-off from 3 to 4 ... and a late lottery pick and a near-second rounder isn't enough to get me to move.
ETA:
A separate issue that you raised is Sexton's role. I don't see him as a PG, not at all. He's a SG who had the bad luck of being born 6'2". He's not going to suddenly learn PG skills in his second pro season. He needs to be paired with either a true PG (preferably a taller one) or a forward who is a de facto PG (not naming any names, but the guy who used to wear #23 here is what I'm thinking). Square pegs never do well when you try to force them into round holes.
Bol Bol isn't quite the #4 guy on my board, but he's the kind of guy to think about in this scenario, IMO. You have to acknowledge that he has less of a chance of reaching his ceiling than Morant, but his ceiling is plenty high, and that's important.
Then, looking at the Suns 2020 pick, I think #12 is close to a worst-case scenario for that. The Suns are a terrible team, and rookie PGs, even good ones, rarely have a positive impact right off the bat. A realistically, Morant's season will probably look something like Young's rookie year, where he puts up big numbers but struggles on and off with efficiency, turns it over a ton, and gets obliterated on defense. In the long run I like him a lot, but it would take a minor miracle for him to carry the Suns to 30+ wins next season. Given that, why would we let them top-5 protect their pick? I agree, that part of my scenario was dumb. Top-1, or top-3 at most would be more reasonable.
As for Sexton, plenty of SGs develop into PGs and become much more valuable because of it. Curry was averaging a modest 2.9 assists and 2.6 turnovers per game in college at the same age. Lillard at age 20 averaged 3.3 assists and 2.8 turnovers per game in an injury-shortened season. If the teams that drafted them had chosen to develop them as SGs, I highly doubt they would've reached their ceilings. It's not obvious to me that Sexton has the same degree of untapped point guard ability, of course, but I think it's a good idea to push him to develop those skills and see where he ends up. His long-term ceiling is much higher at PG than at SG.
The answer is no to that trade because it probably works out much better for the Suns.
You are punting on a top 3 prospect and the extra pick ( bucks pick is probably a throw away) from the Suns is a huge unknown with the lottery system. You are tading morant for a lesser prospect this year for a chance to draft an extra role player next year? Just take the player who could turn out to be a super star this summer. This is not the Nfl draft ! When this team has a shot to draft premium talent they need to run to the podium, plain and simple. There will be plenty of chances to draft role players outside the top 3 in the next decade.
Well, of course, sometimes you can draft stars outside of the top-3. Even in this draft, which appears to be fairly talent-starved outside the top-3, I think there's a decent chance you get at least one unexpected star. Dropping from 3 to 4 probably drops you from 30% to a 10% chance to get a star, but not all the way to 0. Along those lines, I'd say that the Suns 2020 pick is also worth at least a 10% chance at getting a star, maybe more if you consider the flexibility it gives us to trade up with our own 2020 first rounder to get the guy we want. Of course, based on the numbers I used, you still have 30% > 20%, but my point is just that it's not completely black and white which option gives us a better shot at drafting a star.