This sounds very much like the early days of a DPF. In the end manufacturers came out with software that allowed that allowed the vehicle to regen at lower speeds and to cope low mileage drivers.
Decelleration is more suited to a GPF as it increases the amount of oxygen, which raises the temp of the GPF to 400-700c, igniting the any soot in the GPF. If these conditions cannot be met the engine management system will alter the timing to allow the engine to run lean theoretically. The upstreem or downstream sensor could be missreading out of tollerance or any other sensor that needs to be satified to a allow a regeneration.
I have the Tuscon with exactly the same engine as yours and never had an issue with the GPF. If I did to the extent your having, knowing Mb will only pay for one forced regen. I would be telling them I want an early termination long before now.