Its possible to pass through 8 channels from ADI-2 Pro's ADAT IN to its ADAT out, but actually the device is only able to work with 2 channels out of the ADAT data stream, see also manual, its documented there.
What you want to do, to get 8 Mic channels, is not possible with that unit.

Solution: get a RME recording interface with ADAT i/o and connect Mic preamp and ADI-2 Pro digitally.
The least expensive solution for this is to get the Digiface USB:

PC +------- Digiface USB +---------------- ADAT1 IN----------------------------- ADI-2 Pro----------- Speaker / Phones
+                                   +----------------- ADAT2 IN (opt. OUT ++) -------- ADA8200
|
ARC USB

++) optional: if you want to route audio to the ADA8200
Digiface USB as Clock master, ADI and ADA as clock slave getting clock via ADAT

See also my blog articles:
https://www.tonstudio-forum.de/blog/ind … our-Setup/
https://www.tonstudio-forum.de/blog/ind … ort-DE-EN/

Other advantages of this solution:
- by adding the Digiface USB you also get some additional comfort by TotalMix FX.
- you are able to get the optional ARC USB which is very comfortably to work with and remote control TM FX control room
- you have still a lot of ADAT I/O free

You can e.g. use a 15m TOSLINK cable to make a connection to a DAC in front of your HiFi and use HiFi as 2nd Monitors,
to put an example. This is possible if you have either an HiFi Amp with optical SPDIF input or you place an ADI-2 DAC FS in fron of your HiFi with a TOSLINK switcher, then you can use the ADI-2 DAC there for performing high quality D/A for multiple devices: e.g. from computer (as Music Player), from TV, from BluRay Player.
See this Blog article: https://www.tonstudio-forum.de/blog/ind … her-EN-DE/
Not fully up-to-date as some devices changes, but basically what I do to connect Studio and HiFi corner in my livingroom.

This setup is not complex, its the usual thing.

With the Digiface USB you get simply a recording interface with digital I/O which allows you to expand the solution
by Mic Pre's, AD/DA converters and to make connections to the "outside" like HiFi.
Much more important, now you have a "real" recording interface with TotalMix FX support !!!
Setting Clock master and slave is also a usual thing which is required as soon as you deal with digital connections.

So tbh .. I do not see anything in terms of "more complex".
The contrary is the case.
With the ADI-2 Pro alone it can become challenging to catch the exact routing, that you want and
you have no fine grained control over routing like with eg TotalMix FX per port based.

By this Digiface USB / TotalMix FX based setup you come into the comfort zone which allows for more smooth operation via PC, especially if you add the optional ARC USB to this solution.

This is the foundation for the usual powerful RME setup including TotalMix FX.

By this you are able to use the full feature set, that a RME solution can give to you
- TotalMix FX
   - Routing, Loopback Support
   - Snapshots, Workspaces to store and recall settings easily
   - remote control functions
   - efficient code, low latency, no lag
   - IMHO the best routing software that has ever been written so far
- TotalMix FX Remote
   - to be installed on a remote PC in your (W)LAN to remote control TM FX
- ARC USB
   - simply to be connected by USB to the PC where your Digiface USB is connected to
      TM FX performs the interfacing between the remote control and the recording interface

Also of course DIGIcheck, software analysis tools, but these were also useable by the ADI-2 Pro alone.

This is of course rubbish, I overlooked it. Good that you point it out.
I concentrated on presenting him a not too expensive solution where the "desired" ADA8200 at least would work.

The ADI-2 Pro is not a "Mojo Box" enhancing your audio quality.
It simply records audio in a transparent "non coloring" fashion "as is" in best quality.

Of course the ADI8200 has to perform A/D conversion 1st, before being able to send audio digitally across ADAT to something like a recording interface.

ADI-2 Pro FS has only some fixed routings because its more a converter than a recording interface.
To connect 8ch Mic preamps is out of scope.
Without TM FX you are not able to create any customized submixes on the PC for phones or speakers.
What I told you is the best way to get ADA8200 and ADI-2 Pro working and without having to spend much money.