If I add a Montage over USB (so that I get all the channels etc.) I expect that I won't be able to use my RME anymore

If MBP still stands for MacBookPro, then you can use both the Montage and your RME to deliver audio to your DAW. You can construct an Aggregate Audio Device so that the software can look to one place for audio inputs. The Mac is able to gather multiple audio sources and construct a single "Aggregate Audio Device" that includes all of your audio inputs as a single selectable entity.

I don't know the specifics of the RME interface but that should not matter. There are a few things to consider and solve for this to become your workflow.

_. Consider that setting up this many inputs does use resources, therefore creating inputs that you are not necessarily using can affect overall performance (CPU load). Most DAW have a way for you to create custom setup templates that can be catalogued and recalled on demand. This way you can activate just the number of channels you'll need. Mileage here can vary according to the DAW, in some you're reserving the resources when you create the input. Setting up 32 inputs from the Montage is really only necessary in the rare occasion, most often you will be using somewhat less than all 32 at once. Same probably goes for your RME interface... if you are using just a few of its channels while others are unoccupied you probably can reduce CPU load by efficient use of inputs... so creating different configurations of Aggregate inputs is a consideration. They are easy to make.

_. Consider how many and what types of inputs you will be recording simultaneously and customize your setups efficiently. In many cases, if it is a one musician setup, you don't need to configure everything at once. But if more than one performer is involved or if you play and sing, simultaneous becomes a requirement and is worth spending the resources.

_. in the case when you are recording both the Montage and via the RME interface simultaneously, you will be able to record both directly/simultaneously to your DAW, but you will need to setup an analog connection between the interfaces so that you can manage "monitoring". For example, if the RME is the interface connected to your Monitor speakers, you'll need to establish an analog connection (strictly to monitor) the Montage (this connection is for direct monitoring only) pre-record and during record. Without such an analog path to your speakers you will need to use headphones to hear the Montage.
If the Montage is the Audio Interface that is connected to your speakers, then you will need to take the analog L/R Output of the RME to the A/D input of the Montage (as Interface) - again, just for monitoring purposes, you would not assign the A/D to the DAW (in would only be directed to the main L&R Outputs for monitoring purposes.

Hope that helps.

Thank you for your reply!
I have heard of the possibility to aggregate audio interfaces on Mac but didn't use it yet. I didn't expect it to make sense as my aim is to get low latency and stability - which is why I paid so much for the RME with the good drivers...
To simplify my Live-Setup, I would prefer to replace my RME with the Montage. I would send Montage's Sounds as well as my Sounds from the DAW on the Main L/R. I could use the assignable Outs for a Clicktrack to the drummer and maybe a Mono-Version of my Sounds to only be routed to my Inear-Mix. Else, Monitoring isn't an issue as we use a digital mixer and I control my Mix over Wifi.
Is the ASIO driver and audio interface hardware of the Montage good and stable enough to handle this?

Absolutely. And everything will sound awesome through the Montage!

Thank you! I will try out then 🙂