Maybe I'm not clear on what you are suggesting, but...
The 'tracks' are in the sequencer, and I see no inputs what-so-ever in the sequencer tracks for either instruments or audio tracks.
Reason already does record audio directly to the sequencer.
There is a 'mix' channel device in the rack for each track in the sequencer.
The mix channel for an instrument device has audio-in in the back to which the instrument's audio-out is patched - that is, the data in the sequencer track is midi, which is played by the device, and the resulting audio from the has to be routed out of the device into the mix channel.
The mix channel for an audio track doesn't have audio-in because it is already audio - that is, the data in the sequencer track is already audio.
The reason there's a mix channel in the rack is so you can add insert effects, CV control, side-chain - and for consistency across all tracks.
It seems you are saying that you don't want rack devices for the audio track mix channel(s)?
If you want to 'keep the rack clean' you could move all of your audio track devices to a rack column to the right and just never scroll to it. They'd be there but you wouldn't see them because they'd be off your screen
As for saving RAM, the functionality contained in or provided by the mix channel rack device can't go away. if you eliminate the actual device from the rack, or from view to the user, the code still has to be there and it will still need to consume RAM.
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