Unfortunately, perfectly working combination of Digital Mixer + Sonar does not exist.
The only perfect motorised mixer/daw controller system I ever used was the Yamaha 01x, but unfortunately:
(a) it's been discontinued for a long time and;
(b) it doesn't work natively beyond Vista 32 bit
(c) it's really fussy about firewire chipsets
In saying that, I'm a member of a 01x facebook group and a good number of the members have it working on fine on Windows 10 64 bit (they've even posted a video to show how to set it up)... and you can pick these things up for less than £150. Personally I'm happy with my Mackie MCU & Scarlett 18i20, so I've not bothered messing around with my main setup to get this working. The 01X does however work perfectly on my old laptop running Sonar 8.5 & Vista Business 32 bit, and is a dream to use.
I'm seriously considering picking up a Yamaha 01v96i to replace my first generation 01v. I know it sounds weird, since even the newest version of this board is now seriously long in the tooth...but I've grown very accustomed to the work flow and would like better converters that go to 96k in addition to inserts and 16 simultaneous channels through USB. 96k seems to be the sticking point...since any other more recent boards in this form factor want to now stop at 48k. (Behringer/Midas threatened to go there, but they have dropped the ball.)
Does anyone have experience using the 01v96i as a control surface for SPlat?
The Yamaha 01v96i is a good choice, but as you say it's quite old - but AFAIK Yamaha are still selling it. The lack of prominent transport controls and a jog wheel are a bit of a pain. I'm personally not a huge fan of Yamaha pre-amps - nothing wrong with them really, but they're a bit clinical for my taste (also my only gripe with the 01x)... but if you're happy with your existing 01V then you'll be fine.
I guess the choice boils down to how much you need an outboard mixer, and how integrated you really need it.
If you're happy with using the mixer for recording and then letting Sonar take over once it's recorded, I'd suggest:
1. Use any digital mixer with an ADAT output for recording.
2. Use something like the Mackie MCU / Behringer XTouch / Icon / Presonus Faderport for mixing thereafter.
If your really need a more integrated setup, then current choices I know of are the Yamaha 01v96i and Behringer X32 - but neither are what I'd call "fully" integrated.
If you're happy with discontinued gear, there's the Yamaha 01X which has the most complete integration I've come across (assuming you can get it working ok on Win10 64bit) and Behringer DDX3216 with ADAT option (not nearly as integrated and beware of PSU failures on this one).
However if all you're wanting is a tactile controller and can forego the motorised faders, the Kork NanoKontrol Studio is very cheap and works a treat.