You are looking for a MIDI controller to use Ableton as a mixer? I was thinking about the same setup awhile back and came across these recently: https://intech.studio/
A bit pricey, but what I have in my head is getting 6 of the PBF4 or EF44 modules. That would give me 24 faders that I could hard assign to each track.
wow these look amazing ! But yeah, sadly, a bit too high for my budget. (Why did I buy a Digitone ?!!!)
that’s exactly the kind of stuff that would fit my needs and they look really well build.
It should work.
In my current setup, I have Ableton sending midi clock to the Digitone through USB (sync turned off in Overbridge as above). I have the Digitone sending transport and midi clock through a midi cable to my Dreadbox Typhon. It seems to be following the tempo just fine. That seems analogous to what you want to do.
this would be a really good news 
So I have a question about latency - for live performance, what is an acceptable time? I’m considering using a drum machine and looper and then mixing these in Ableton via an audio interface (MOTU M4). I would be using a few FX on these 2 (delay, saturation, delay and reverb, mastering chain). that I tweak via a midi controller.
It depends on a lot of factors but generally I try to stay in the 10-12 ms range, total. Ableton will help you keep all your tracks in time but the latency would be noticed when tweaking knobs on midi controllers, etc.
It’s unfortunately a complicated matter when you start adding plugins and routing things around. This video might help you. I’m on this journey as well, reintegrating Ableton into my live setup.
I also wanted to chime back in this thread rather than create a new one. Specifically on the topic of gain staging. The individual tracks obviously come in quieter to accommodate some mixing headroom, I think at about -12db. I still generally do my mixing on the hardware itself, so would be curious to compare setups here.
Just run a gain boost across all tracks?
That’s what I do. A 6db boost with the gain utility on any overbridge track, if I remember correctly.