I dont think the platforms are long enough for the 5-car 730s. So I would assume 350s would cover it. I know there was a project at very early stages (GRIP 2) which was reviewing the feasibility of through trains to London as well as a passing loop for 2tph. This was 2016 so I'm not sure how far this got.
The Abbey Line is well-used, but a more frequent service, a timetable with regular intervals and a through service to London would be very beneficial.
I would divert all stopping services from Euston (to Bushey, Harrow & Wealdstone and Wembley Central) to St Albans Abbey (2tph). Then have 2tph from Euston to Milton Keynes Central, running non-stop to Watford Junction then all stations to MKC.