I need a buck controller recommendation for 600W

Hello David,

First you should look at various topologies and decide which may be most appropriate for your design requirements.
Buck, Push Pull, Forward,
Here is a great tool for various topologies.

POWERSTAGE-DESIGNER Power Stage Designer™ Tool of Most Commonly Used Switch-mode Power Supplies | TI.com

UCC256303 is a LLC resonant controller.
While it may be applicable for your needs, you must first decide on topology, efficiency, ripple, etc, before choosing a controller.

At 600W output you should consider a multiphase buck topology.
That is several buck circuits arranged in parallel to share the load and minimize ripple current and voltage.
Here are some application notes for guidance
Multiphase Buck Design from Start to Finish, Part 1

Benefits of a multiphase buck converter

TI's Reference Design Library has several examples of how to achieve high power buck's.
      Reviewing these will help you understand the various methods and complexity.

35V to 60V Input, 600W Output Dual-Phase Buck Converter Design with Greater than 97% Efficiency
24Vdc Input (Double Car Battery) Automotive Multiphase Sync-Buck Reference Design
450W High Efficiency 25V Buck Converter Reference Design

The  Reference Design Library  also shows many other topologies in the 600W stepdown range.
Using the various filters will allow you to see many possibilities.

One concern is going to be your duty cycle maximum.
30Vin and 29Vout is 97% duty cycle.  
This will limit available controllers and may not be practical.
And 20 amps of output current adds to the duty cycle concern.