I traveled on a Liverpool to Brum service today at around 3 pm, it seemed to be ok with 8.350, but I have been on some very busy Liverpool services in the past too. I do wonder whether 730s will run Liverpool's, considering that most LNR routes through Birmingham will remain 350s, it just seems peculiar that these would be run as 730s, perhaps it would be beneficial at peak times? If 2tph didn't seem to be a dead plan, a single 5-car every 30 minutes could work, there are enough LNR units to run the service.
If they are to be used on Euston-Tring/MKC, Euston-Crewe, and Euston-Liverpool, and if every service operated as 10.730, that would be 26/36 units in use each day, if my calculation of the number of diagrams is correct. I'm not that well educated on how much money there is to be saved by coupling and uncoupling to meet demand throughout the day, part of me thinks that it may just be easier to keep them coupled throughout the day, as is done with 8.350s currently.
I do wonder what the future of the 350 operated services will be like in terms of unit formations though, as it seems that there will be many more 350s out of use each day, considering Euston-Birmingham requires 22 units currently (11 8-car diagrams), with the Abbey Line requiring one, and Birmingham-Crewe requiring around 3 or 4 (though I'm unsure as to whether this will be run by 730/0s in the future, as it is displayed as a WMR service on the PIS, but is also branded as an LNR service on route maps), meaning only around 27/50 units would be in use each day, could we perhaps see the bolstering of some services to 8/12 car? Euston-Birmingham is healthily loaded, and less frequent than prior to COVID, though I understand that there are very few stations north of Northampton that could actually handle 12 carriages (Rugby, Coventry, Birmingham International and Birmingham New Street are the only stations if my memory serves me correctly, leaving Long Buckby, Canley, Tile Hill, Berkswell, Hampton-in-Arden and Marston Green, the latter only gets a Euston-Birmingham service on Sundays at 1tph). I wouldn't be surprised if the DfT didn't allow this though.