Ah yes, have seen this before. Repaired several of these transmissions.
You are losing pressure. There are five seals between the Mechatronics valve body and the transmission clutch assembly. These seals need to be replaced. They age, become compressed and the main seal can tear, causing no gear engagement at all.
There are some other symptoms that are indicative of solenoid issues, but sounds more like a pressure problem for you. A hard shift into first gear when slowing to a stop, can be a solenoid problem.
If you have a shop do this, expect a very large bill. Some shops will not do this service and want to replace the transmission, higher profit.
It is a little more fiddly to replace the seals, but not too difficult. A pair of long 90° needle nose pliers are a must! See video link below.
Here is a photo of my last 6HP bridge seal, that would not go into gear at all.
This is the complete kit.
https://www.fcpeuro.com/products/bmw...24152333907kt3
Please verify fitment to your particular application. Without the VIN number, I cannot be certain this is YOUR kit.
How to video. Includes trans service and fluid filling procedure.
https://youtu.be/2g-5uY_P7uA?si=CI5Dxw4RpRNl6s04
After the third or fourth one, it gets easier. 