it was hitting the thermal limit nearly the whole time
You are not alone. A 14900HX running a -150 mV undervolt still needs 230W to run at its full rated speed. No laptop manufacturer has been willing to face up to that fact. You end up having to choose between power limit throttling or thermal throttling or constantly alternating between those two. Dell dropped the ball but so did many other laptop manufacturers.
Your cooling system can manage about 160W but only for a second or two. Long term it is more like 125W. You can set PL1 to 130W and PL2 to 160W. I would reduce the turbo time limit down to about 4 seconds. Intel recommending 56 seconds of turbo boost time in a laptop with a 14900HX is wishful thinking.
If you ever pull your laptop apart you can try using Honeywell PTM 7950. This will not likely make much of a difference unless the thermal paste application on the assembly line was really botched.
Poor cooling is reducing your CPU speed by almost 1600 MHz during Cinebench. Manufacturers should be building their laptops twice as thick so they could include adequate cooling. They must think that nobody would buy something like that. In theory next gen HX laptops should have similar performance while producing a lot less heat. I have not had my hands on one of them yet.
You should update to ThrottleStop 9.7.3. The mV Boost feature was replaced by the new V/F Point feature. Adding 50 mV to the 5 highest V/F points allows me to run the Cinebench single core test at nearly 5800 MHz for the entire test. Setting V/F Point 1 to 150 does the exact same thing as setting mV Boost to 150. The new log file separates the P and E core data so you can have a better look at what each is doing.
Attached is the log file from a few Cinebench R23 runs I did while plugged in outside in cool air. There was no throttling during any of the three runs. First run was at default multipliers, 52X and 41X. Second run was with a P Core overclock of +100 MHz and the last run was with both P and E cores overclocked +100 MHz. A laptop with a properly engineered cooling system should run like this all of the time without having to go outside while wearing a winter jacket. Good luck trying to find something like that.
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