Lots of iron filings after flush. Overheat troubleshooting.

[Double E]

'99 Cobalt 253 7.4 Volvo DP. Open cooling, fresh water use only. 350hrs.

I hit a big log on the lake, bent up the props, got them fixed, reinstalled a few weeks later and went out.
Since then, when on plane at about 30mph, the gauge goes to 190 and will keep climbing until the throttle response is cut (I'm assuming it goes into protect mode automatically). No alarm sounds but I'm, not sure this unit has one either.
Took it home and looked at the impeller. It's perfect.
Went back out the next weekend and same overheat situation happens. This time, I take the temp gun and temps all match the gauge near the t-stat. Risers are hot to the touch and I can hear water boiling inside the block. I pull hoses while idling and they're all moving water.
Back at idle it stays cool, 170-175.

It's late Fall, so I resolve to begin winterization, Water coming out of the lower unit flows fine with all hoses connected and being fed by a hose T into the water system, near the impeller.
This time, I decide to force a garden hose into both block drains with all hoses at the t-stat and impeller disconnected and out comes an ounce or 2 of small rusty iron flakes. My stomach gets queasy and I call my mechanic. I cannot get it on his schedule till January '26 and once there, I explain to him all of the above and show him the flakes.
He does the lower unit service on it, determines it is fine, checks the t-stat and runs it in his shop on a hose. Says it all seems to be fine and finds no issues, shows me pictures from his borescope that reveals normal wear internally through the risers and everywhere else. One other thing he mentions was that this ECU will store codes, even after a power disconnect. It had none stored.

I'll be putting it back on the water in late April but I'm looking for anything else that could cause the issue.
Perhaps the dislodged flakes were the issue and it's now resolved, but where did they come from?
Maybe the oil cooler assembly? I do not think my mechanic pulled it off to check it. If it is not sending water into oil or vice-versa, could it still be the issue?
Weird that all of this began to happen after hitting that log.
Open to your thoughts on troubleshooting the when I splash it in a couple of weeks.

[Kate]

"I hit a big log on the lake . . ." ". . . overheat situation happens."

I did the same thing.

Did you check the prop shaft? There are a number of ways that a bent shaft can cause overheating.

[Double E]

Shaft appears fine. The props came off and went back on with no binding if that is a reasonable way to assess a bent shaft.