So today the schematics got an update. This should be more or less final form for the moment, no more feature creep.
Next thing on the schedule is designing PCB which will take a bit of time since I want to stick with single layer design friendly to home PCB manufacture.
The features now include attenuator, AC coupling with offset, DC offset, some safety, two LEDs for status...
As before, this is still in development and hasn't been tested, so I don't know whether it actually works, but to best of my knowledge, it should.