25HP Mercury BigFoot 2001 overheating even after replacing impeller with kit and removing thermostat

Hi everyone!
I could really use some help here, I've got a 2001 Mercury Bigfoot, 25hp 4 stroke that's overheating after 3ish minutes if I run above an idle.
It's running on a fairly shallow sandy river.
Removed and replaced thermostat, no change, still overheats.
Ran the motor with thermostat and thermostat cover removed, my question is, how much water should be coming out of this opening?
Here is a link showing the motor running without the thermostat cover on.
https://youtu.be/2dU_bg85oVM
Any ideas what my next step should be?

Thank you very much in advance for any help you can give me!
Other steps taken:
Removed lower unit and inspected, impeller, looked to be in good shape but one fin may have been installed backwards.
Reinstalled the impeller, no change, still overheats.
I've replaced the impeller and thermostat.
Installed it following instructions here
https://youtu.be/cKD26YcZPT0?t=400
Still overheats

I removed the exhaust cover per page 230 of the below service manual, there was a tiny dusting of sand in this, I then shot compressed air everywhere I could get to under this cover.
Found service manual here
http://forum.rentboat.dn.ua/manuals/merc/25.pdf
Still overheats....

It looks like plenty of water. Your pump is working great Does water come from pee hole?
Blow or squirt onto the pee hole opening with thermo cover off. See if it open on that side.

I hooked up the hose to the pee hole, no obstruction there.
I removed the exhaust cover again, pulled the lower unit off and tried to push a wire up from below, was unable to make it, there seems to be some obstruction 6" - 8" away from where I would expect to see the wire come out under the exhaust cover.
I then tried pushing the wire from the top and hit an obstruction about 4" down. Is there any kind of screen or something in that area? Or does the pipe bend quite a bit there?
As a last ditch effort, I hooked the garden hose to the pee hole and re-installed the exhaust cover without the thermostat installed. The water is pouring out the exhaust but only has a slow flow out of the pipe coming off the pump.
While the hose was connected, I then tried blocking the flow coming out the exhaust but even with the increased pressure, I couldn't get a hard stream to come out the fill pipe... It seems like there is some type of obstruction in that pipe. The other observation I have is the screen covering the inlet is missing on this engine. (Part 43 on page 365)!

Followup, we ended up getting it working!
Removed the lower unit, then hooked a garden hose to the flush out port on the telltale and turned it on. There was a weak stream coming from the water pickup tube.
We then purchased something like this from Ace hardware
https://www.homedepot.com/p/Cobra-T...710/307740982?source=shoppingads&locale=en-US
Cut off 4' of it and hooked it up to a cordless drill. We then used the snake and drill up and down the pipe. We put a bucket under the stream to watch what kind of debris was coming out. It looked like mud, sand and then quite a bit of black rubber material. The stream got stronger the longer we did this.
Once we noticed a silver fleck or two (I'm assuming the aluminum block) we stopped immediately and put the lower unit back on.
We ran it for an hour or two while checking the oil to make sure we didn't somehow get water in the oil.
It DID overheat when we ran it at full bore for 15 - 20 minutes, so we backed off the throttle and never had a problem since.
I don't know if this is the correct procedure, but it seems to have worked for us and should get us through the season until we can work on it in the shop.
We also ordered and installed the screen covering from page 365, part 43. This was missing when we received the boat.

quick edit: We also ran it with the thermostat cover removed and after this procedure there was at least 2x the amount of water going through as before in my first video.

The black rubber might have been impeller pieces.

We ended up removing removing head and lower unit, found a gasket that was almost completed crusted up from age. Replaced this and it's running much better.