I got that sound mixing on 2 02r's for years

You have better gear now than the 02R. If you can get it right with an
O2R then you can get with the MOTU.

I think I am running out of headroom.....That kinda thing....But if I back down everything, I
effectively raise the noise floor, and reduce resolution....so external
summing allows higher resolution, while giving me even more headroom.

24 bit digital has a dynamic range of more than 144dB. More headroom
than analogue.
Maybe you ARE pushing too hard. It's ok if you mix only halfway up the
meters. That noise floor is way down!

See if you can hire some coverters and summing boxes for a weekend.
Then you'll know if it's worth spending thousands on gear that may or
may not give you what you are looking for.

Regards

Chris