There's quite a few videos on Youtube that show how the transmission works and how to diagnose the problems. Here's the one I used to rebuild my K46. I've never done one before but after watching several different videos I decided to do it (same as yours). I only paid $25 for the mower so there wasn't much to lose and a lot of money to save. There's other options for oil rather than using the TorqTuff brand that's much less expensive. There's a lot of videos also on Youtube that will show exactly how the transaxle works and maybe you can spot the problem.
This is a good video once you get past the theatrics:
Here's the Ebay seller that I sent the parts to to do a refurb on the parts. If yours' aren't able to be rebuilt, he'll supply the parts you need to get it working again. I think I paid $177 for all the parts I needed because mine were gouged and scored and had several broken springs.
Discover the unparalleled performance of Tuff Torq transaxles, the world's leading choice for lawn mower tractors. That's when it's crucial to rely on a professional. Don't forget to check for a label on either side of the axle, as most transaxles have one.
www.ebay.com
Here's the fluid I used:
https://www.amazon.com/dp/B08XN5GJ3K?psc=1&ref=ppx_yo2ov_dt_b_product_details This came highly recommended by a guy that does this for a living.
Cleanliness is extremely important when working on these so find a good place to do it. Take pictures, starting with how all the linkages connect and what the transaxle looks like after taking it out of the mower. Power wash the whole thing if you can. The better you clean the exterior, less chance of dumping debris into the case. Then take your time disassembling and cleaning, reassembling.
Here's the follow up thread after I got it working again. It's been working great ever since. Will probably do the same to my YTH 42" also next spring.
https://www.lawnmowerforum.com/threads/update-on-25-yth22v46-riding-mower-project.73590/
Let me know if you have questions and I'll try to help.