That's the best phone mount solution for the gen 4 Sienna I've seen yet. My only suggestion would be to move the iOttie holder higher on the ProClip base to provide a clearer path to the emergency flasher button and to somewhat reduce having to look away from the road. . Quickly hitting the flasher button without thinking has saved me from several rear end crashes when highway traffic ahead had suddenly come to a stop.
The attached photo showing the phone position I'm talking about is from the ProClip website.
You might be able to eliminate the 1" to 17mm ball adapter by using other methods. For example, attaching a Pro.Fit extension bracket with a 17mm Pro.Fit "Bravo Ball" to your ProClip base could raise your iOttie phone mount and eliminate the 1" to 17mm ball adapter if there is enough clearance between your iOttie holder and the Sienna's infotainment screen. The Pro.Fit extension bracket is heavy rigid plastic and can be cut with a saw or Dremel if necessary. The holes in the extension bracket conform to the AMPS spacing standard. Or maybe you would prefer using the Pro.Fit "17mm Ball Adapter Plate" which could also eliminate the 1" to 17mm ball adapter you are currently using.
Here's a link to the Pro.Fit accessories page: Accessories.
iOttie phone holders are pretty nice. We have Qi wireless charging versions of them in our Sienna and Prius. You might be able to go with a wireless Qi iOttie phone holder in your Sienna if you get a USB to wireless CarPlay adapter.
I have a similar setup that ive used in other cars. I bought the proclip base and 4 screws from proclip, a 17mm ball on an AMPS plate from ebay, and the same iottie from the pics in the first post, which goes directly onto the 17mm ball once you remove the air vent mount.
i prefer a lower position and have the AMPS plate on the lower half of the proclip base, but your point about not blocking the hazards is a good one.
i have tried the iottie and other manufacturer wireless charging mounts in the past, but found them to be inconsistent, and the sienna uses wired carplay anyway.