I have my oil temp sensor in the filter head. I don't think that's a bad place at all. There is an o-ring fitting by the ECM that would give middle of the block temps, but the adapter to get from o-ring to NPT would likely remove the probe from the flow giving a better chance of false readings.
Just because it's right after the cooler doesn't mean it's not a good reading. We are really only interested in knowing if something is abnormally high with the oil temp, and it will certainly indicate when the oil cooler is saturated.
I don't have too many miles on my oil temp gauge yet, but will change soon at a GCW of 20K, but what I have noticed is that oil warms up linear to coolant, but about 10° cooler when not towing and 20° cooler when towing.
I've also noticed that oil takes longer to cool down that coolant after pulling a hill, or even just driving.
In level cruising empty the oil is a few degrees above coolant, but towing it's about 5-10° hotter on level ground. No big grades yet.
That data tells me that the oil cooler is certainly not 100% efficient and won't overly skew the oil temp readings, at least not enough to indicate if there is an issue or not.
I have found that the max allowable oil temp for the ISB without an EGR is 248°, but I haven't been able to determine where the OEM probe are located.... if there are any. If there aren't any ISB's with OE gauges then I'm fairly certain the test ports in the filter head are used for readings.
My dad has a 2017 with oil temp, but I haven't been able to play under the hood to see where the sensor is. I've read it's all an algorithm, but it doesn't act like one when I drive it. I've also heard there is a new sensor on the block near the oil cooler. His warm up and cruise temps look very similar to mine.