Sorry, but this is not going to help you that much; I'll just describe how I record stuff with Reaper.
I don't worry too much about the input level in absolute terms, as long as I'm seeing a decent signal on my on-screen graphics. Different instruments, and different ways of playing can produce different levels, and any imported samples or clips will have their own levels. Before mixing, however, I usually 'normalise' all tracks, which makes them 'compatible', each having the same peak level (can be 0db; I usually prefer a bit less, so normalise to -3 or -6 db...). In the past, with analogue, tape systems, this could increase background noise (tape hiss and the like...), but with modern 24-bit recording, this is, in my view, a thing of the past. I prefer recording more on the low side, then normalising, rather than recording 'hot'.
I'd recommend trying this with your system, except that, in looking through the features of Bitwig, it would appear that they (deliberately..?) don't offer a 'Normalise' treatment..! Seems odd to me, and they doubtless have their reasons, but I'd be a bit stuck without it for mixing tracks with disparate levels. Still, that's how I deal with low input levels. To me, they're preferable to 'hot' levels (within reason, of course..!). As stated, this might not help much, but that's how I deal with things. There's nothing wrong with your interface; it's fine.