I have only used LGB and Massoth decoders. I am not familiar with MFX.
If you have only one Loco with an MFX, have you considered replacing it with a regular DCC such as the 8154401 eMotion. (replace the LGB 55021)
MFX is a Marklin protocol that is included in the modern LGB decoders and has been for at least 8 years now. The decoders in LGB locos since 2014 are called MSD3 decoders and are multi protocol, meaning they support MFX, MM, and DCC formats. It is completely unnecessary to replace this decoder as it functions just fine in DCC.
It is recommended if you do not have an MFX-capable system (only the Marklin CS3 and ESU ECOS currently support this format) then you turn off the unused protocols as the decoders have a protocol hierarchy, and MFX is at the top, DCC at the bottom. As other stated on these decoders that can typically be accomplished by setting CV50=2, though Marklin has changed the protocols and settings numerous times on the MSD3 decoders, so check the manual for you loco on the exact values possible. CV50=2 will typically make it DCC-only.
If you buy a Massoth CS 1210z it comes with the Massoth Programming Tool or MST. It will allow you to read and write to Massoth decoders only, however. I do not believe other brand decoders are recognized. You can very easily read and write the few CV values you would be interested through the native programming function of the CS and Navigator regardless of decoder brand. The MST will allow you to update the firmware of the various Massoth components, however, and is a useful tool for that.
For those coming from the older LGB MTS systems Massoth is a great logical upgrade for DCC control. Massoth produced the electronics that were used by LGB prior to the Marklin takover and its format is pretty intuitive and easy to use. It provides most everything a typical garden railroader would require and plenty of power.