Hello Ben
Sorry for being a "oiseau de mauvaise augure" (bringer of bad news).

Here is the little I know:
- The Märklin 6021 digital languages are Motorola and MM2 (the enhanced Motorola) issued in 1995.
- I cannot say anything on the Intellibox 1. Is it DCC-capable? The 65150 is. Is it low voltage- capable? I don't know
- the Digital central unit must be DCC-capable, accept low voltage required by Z trains (closer to 10 V than the 19 V of the Märklin ones)
- the key issue is to digitalize locos. A German manufacturer (VELMO) provides replacement PCBs to digitalize some Märklin Z locos. These are using DCC and require no machining of the chassis

Look here for information https://velmo.de/html/english.html
I suppose that preference must be given to large Diesels or Electrics
An extra clean layout is needed so a tight cover must be considered.

I am a big fan of Märklin digital and PC control (Rocrail) so yes, digital brings a huge improvement to your layout.
Here is mine (sorry HO and MFX)
https://www.youtube.com/watch?v=F6WnhXPcRqI
Cheers
Jean

Thx for sharing the key information for doing Z digital (voltage limiter and decoders). I'll begin with adding catenary to control 2 trains on one track and then try digital. Some of my (vintage Z) locs do not have a decoder option.
Ben