I'm not sure if anyone's got the Modern Railways magazine that has a GWR section in it (the July or August 2019 one) but from looking at that, it looks as though
the only diesel trains to regularly use London Paddington are a Turbo on the first working from Paddington to Greenford and the Class 57's on the sleeper services.
It also says a 12-car Class 387 17:46 from Paddington to Didcot (calling at Reading and Didcot only) will be introduced. It will be useful if the existing 16:36 is retained but a 17:46 I hope people have common sense to use rather than potentially fight for a seat on an IET.
Personally for me a 12-car Class 387 at 17:46 to Didcot Parkway means I can go arrive in London around say 11am.
I'm told and have seen on the notice boards at Paddington by some of the ticket gates (ticket restriction boards) that all Electrostars are Off-Peak).
I wouldn't be surprised if GWR soon start advertising Reading and Didcot Parkway as pick up only in the timetables on say 1 or 2 Peak IET's that might run very close to a Class 387 to / from Didcot, Reading and Paddington.
The earlier train runs on Saturday mornings throughout the year, I believe to connect out of the sleeper. It did run one year during the week, but carried virtually no-one, so was nicked to strengthen services into Plymouth IIRC.
This makes Deepgreen's assumption wrong - the line doesnt cater for commuter traffic all year round, because there isn't a significant flow to serve competitively (Quintrel Downs to Newquay perhaps?).