I would like to thank everyone who helped me out. It was a successful diagnosis and replacement. To highlight a few things along the way
1. Start with testing the 24V and 24V (GND) voltages. The key here was that while it started out with 28V, as soon as the spark ignition kicked in the voltage started fluctuation wildly so that indicated a module failure (on the control unit the voltage was steady/constant)
2. The Honeywell S8610U is the correct replacement module for the S8600M. I did check the manual as well to ensure that the settings are correct. The key things to look at are Pages 2 which talks about the DIP switch settings SW1 and SW2 when replacing the S8600M (different variant models as Yuri pointed out) with the S8610U3900. The default factory settings for the Switches are a perfect match for the S8600M so don't change them. The second thing was look at page 8 which talks about the wiring when you move from the S8600M to the S8610U. It's a perfect 1 to 1 match with the exception of the 24V connection from the S8600M which needs to be connected to the TH-W input instead of the 24V in the S8610U if you don't have a Vent Damper installed. The detailed wiring diagram can be found on Page 13 as well. If you have a Vent Damper installed, see the manual, again pages 8 and 12 (google what a Vent Damper is if you don't know, it's basically a unit connect to your Exhaust Vent and most systems no longer use Vent Damper as they are expensive and ineffective). Here is the link to the manual:
https://www.supplyhouse.com/Honeywell-S8610U3009-Intermittent-Pilot-Control-4584000-p
3. After installing do a heat exchanger inspection, it look good to me with no deformaties, I was able too look at it through the burners section using a high powered torch. You can also do the burner test I shared above.
Just a quick point to note, when installing the S8610U, it's best to install it flat on it's base, there's one side which should never be covered (it's written on it and also mentioned on the manual). Also if you have a separate flame sensor/ignition then you need to cut off the factory installed Wire Jumper to the sensor connector and instead connect your separate flame sensor to it. In my case it was combo ignitor and flame sensor so everything was factory default for me.
The new module has 2 LED lights on it for flame and operation which are very helpful to diagnose issues.
I hope this helps other folks. You can buy the S8610U from Amazon for about $95
https://www.amazon.com/gp/product/B00ECV3H3K/?tag=skimlinks_replacement-20