Tow/haul mode

“but could not hit the brake again as I was high on rpm already” What?

Leave in tow/haul mode. Don’t overthink. Use your brakes as required...slow way down, release brakes to cool, repeat. Read an article on downhill driving.

Have you read your manual?

I’ll bet not, fisherman.

What tow vehicle? Is it diesel or gas? What kind of load are you towing? Give us something to go on, here...

I my case going down a steep grade, with a diesel and a six-speed transmission, I put the transmission in "Manual" mode and select the gear I want. I can then tap the brakes as needed to supplement the engine brake.

Rob

If the TV is anything newer the computer should protect the motor from you. So no it shouldn't down shift again unless you have a 10K redline.
Sounds like you need more TV or less Trailer and to slow down before going down long grades.

Assuming the transmission and engine are working properly together, you can step on the brakes again to further slow down. It won't down-shift again into 1st until you're going slow enough to not exceed allowable rpm's.

You were not correct. Leave it in T/H. Most vehicles I've driven (Chevys) won't downshift to 1st. No matter the vehicle the computers will protect it from over reving. Putting the brakes on further will not cause a downshift to "destroy the engine" rpm.
If I'm on a road I know I like to just use manual mode because I know what gear I need to be in for which hill. On a new route, just leave in T/H and apply brakes as necessary.

Sounds like your dealing with a subpar tow vehicle struggling to slow the mass, honestly.

20 years towing in tow-mode and not seen those RPMS.

Perhaps a manual downshift to mitigate excessive racing of the engine.

Tow/haul mode works in many different ways depending on vehicle brand/which tranny/which engine and a host of other criteria.
Need more input for a accurate shot of how your tranny can work.

“Automatic”...meaning it has a computer managing EVERYTHING because humans messed up with manual transmissions

It will ‘try’ to protect itself and the rest of the vehicle and if you put it into manual mode..some of the safeties will be be disabled or bypassed...except for a few that will continue to protect itself

Read the manual

Agree with others, sounds like you have too much trailer for your TV

Not much help without more information on the OP’s TV and trailer