Engine slows down with blades are engaged?

Check the tension of the deck belt. See owners manual

https://www.manualslib.com/manual/502723/Husqvarna-Mz5425s.html

If the engine has oil leaks from the valve covers or crank seals be sure the belt is dry. If oil.

If belt has burnished it may need replaced.

The engaging of the mowdeck is cause an excessive demand of your engine. From that, we can assume your problem is in the deck. Since the power drive is all about belts and it threw the belts, from your information I can only deduce that either an object (stick-wood-rock) has wedge into a position that has stopped rotation of the blades. Also, each blade has a pulley/spindle. These have bearings which must be free to rotate.
A bearing failure or seizing can also cause the problem you report.

Follow safety rules in your manual to insure the engine cannot start. Attempt to rotate each blade to check for free rotation. If the do not you most likely will need to remove the mower deck and check further. An obstruction or seized spindle should be easy to determine and sit repair is easily possible.

Craftsman Garden Tractor 917.28746 Engine shuts off when ...

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Craftsman Garden Tractor 917.28746 Engine shuts off when engaging mower deck. 917.28746 ... Then start the tractor and pull the pto button out. If the engine ...

Why does my engine die when the blade is engaged ... - iFixit

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iFixitI have an older Murray riding lawn mower, whenever I engage the mower the engine dies, otherwise it runs .... My dr feild brush mower ... cuts off when I engage theblades. ... My Huskee only shut off once I put it in drive with the blade engage.

Riding Lawn Mower Problem - Dies when PTO engaged - GardenWeb

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The brake mechanism works when tension is released from the idler / cable. The brake is spring loaded to contact the spindle this stops the blades from spinning. I have attached a drawing showing the brake arm and where it can be found on the deck.

the lowerer the deck the more your working the mower. either mow more often or reduce speed

check your blades to make sure that they are not hitting the deck,, this would cause the knocking sound

the safty swich is getting hot & wont alow it to engage again untill cool

Depends on how it cuts off. If it gradually slows down over a few minutes or surges, it's likely the carburetor needs to be cleaned and rebuilt. But if it's an immediate cut off, it's likely to be a safety switch such as the one that shuts down the engine when you get off the seat if the mower is running. Sometimes this happens frequently to a light person who bounces on the seat when mowing.

YOU MAY HAVE A BENT BLADE OR EXESSEIVE BUILD UP UNDER THE DECK

It sounds like your blades may be restricted when you lower them. A possible solution for this problem would be to remove the mower deck and clean out all the dead grass buildup inside the housing.