> I would like to use the Tascam because it offers very good AD- and DA-conversion

The device has been introduced 2013 to the market and then you can be sure, that converters have been used for this design, which are already available on the market and which are proven. So I assume that these Burr Brown converters are at minimum 8-9y old. So I do not get this point from you why you want especially this device for AD/DA conversion.

Current RME converters from RME should have better specs.

> and yes, from time to time I want to record some tracks on my very old-school tape deck because
> in our classic car we can only play tapes ;-) And the Tascam really offers various inputs and outputs.

The same is applicable for the UFX II and UFX+.

And I am still wondering, why you want to spoil the around €1000 for the Tascam, IF you intend anyway to get an UFX * with DURec. Sorry, but this does not make any sense to me.

> What would be the problem using electric SPDIF- and AES/EBU-connections like I have shown in my sketch?

1. of all again my point: wastes money for a device which is most likely not required in any way

2. instead of making the UFX II the center of your recording solution you have always the Tascam in between the UFX II and your HiFi

3. because the Tascam does not support optical SPDIF. Instead of this you need to use coaxial SPDIF without galvanic isolation, which is more sensitive to outside electrical influences / hum.

With the UFX II you can do everything.
Use it completely standalone or with a PC/Laptop ....
Store / Recall up to 6 prepared configurations / routing, for every situation that you need.
You can record any input with DURec to normal medium.
You can record with a DAW, with simply a recording program, with Global Record from RME (lightweight recording solution).

You have already two very good phones outputs and plenty of I/O ports that are of good use.

You have two options in terms of D/A conversion
a) use the D/A converters of the UFX II and connect it to the balanced ports of the NAD or
b) use the D/A converters of the NAD .. put maybe this TOSLINK switcher in front of it and then you can
let the NAD make D/A conversion for UFX II, TV, bluray
c) or even a mix of the two ... whatever you want.

You can even use a smaller RME recording interface like BBF Pro FS or UC or UCX without DURec.
Then simply use RME Global Record to record something or a music player.
This you can route to analog outputs and there you can connect a cassette deck to make recordings for your oldtimer.

But whatever comes to my mind .. I see absolutely no necessity for the Tascam device. It makes no sense to me.
I see no advantage by getting it. Or you think you need or simply want such a device.

And even then I would think about simply to connect it to a recording interface like a tape deck.
I would digitally connect recording interface and your NAD by TOSLINK cable (optical SPDIF).
Eventually get the TOSLINK switch additionally.
And then I would let your NAD perform the D/A conversion.
It can perform it surely as nice as the Tascam.
But then you have galvanic isolation in place and the Tascam does not need to stay on all the time.

You should also check what all needs to be connected to the recording interface.
Maybe BBF Pro is to small in terms of analog I/O ports.
Maybe UC or UCX would be better.
Most flexible UFX II with DURec.

Basically this is my proposal under certain assumptions that I understood you correct in terms of old tape deck in old car.

Recording interface is discussable .. but I tool the UFX II, because you picked also a digital recorder (Tascam).

D/A conversion can be done at the NAD ...
or alternatively also by using an ADI-2 DAC FS / Pro FS R BE.

BTW .. you will need the XLR connection for SACD, because SACD audio can't be send via SPDIF (copyright reasons and too limited bandwidth).

I looked into the NAD manual of your amplifier. You are most likely not right in terms of optical SPDIF only supporting up to 48 kHz. Only the two USB ports seem to be limited in terms of bandwidth, USB Type-A supports e.g. FLAC up to 48 kHz, USB-Type B FLAC up to 96 kHz. All other digital ports (coaxial and optical SPDIF and AES/EBU) support 32 - 192 kHz.

The analog ports of the UFX II are "servosymmetric" (see manual 19.1), balanced and unbalanced cabling is automatically detected, depends on whether you connect a cable with TRS plugs for balanced operation or with TS plugs for unbalanced operation. So you can connect many different analog gear as you like. Different reference levels are supporterd per port, etc. The best overview you get in chapter 39 of the manual about port capabilities like supported reference levels, etc:
https://www.rme-audio.de/download/fface_ufx2_d.pdf
https://www.rme-audio.de/download/fface_ufx2_e.pdf

Please note if you would like to connect the Tascam via AES .. AES is bridged with ADAT2, see block diagram.
And ADAT2 is the only ADAT port supporting optical SPDIF...
So if you use ADAT2 for optical SPDIF connections, then you can not really separate the routing between ADAT2 and AES.

to 1), I don't know the tascam, this  is RME forum

to 2) with TM FX you can route any combination of input signals
- from HW inputs (the actual inputs on the device)
- from SW playback channels (audio coming from computer)
to any output

Loopback is also supported so that you can route such a complex submix on an output to the corresponding input of this channel where loopback became activated.
By this you can record such a submix ... also audio from any source of the PC (youtube, ..)

We talk about two different things here

a) hardware design of the unit, with only one speciality that ADAT2/AES are kind of bridged
b) general TM FX routing capabilities

If I remember right .. If you send something to ADAT2 then its also being sent to AES and vice versa.
And of cource you can store routings ..
This doesn't impact the normal routing capabilities in any way, the only thing is that these two channels are not separated.
So with this information you should be able to follow the signal flow of all of your use cases, whether all is possible that you want.