In Dec 2019 the 150 fleet will be 20 x 150/2 and 2 x 150/0, working Exeter area services and Cornish branches. The 158 fleet will be 7 x 3 car and 11 x 2 car, working some Bristol area services, some Cornish main line services and the Barney branch.
What happens after that is still up for grabs as the DfT decide what they want to buy (service wise) in the next 2 year DA award, DA3, which should start next year. With the arrival of the 769 units, there should be several more 165/1 units heading west towards Bristol and Exeter. Also the 2 x 150/0 units are not being offered by Angel to GWR past April 2020 but releasing any more cl.150 units depends on what level of service improvements the DfT buys.