Not quite your situation but maybe a useful data point: I usually do Stroud-Paddington on Tuesday mornings (10:30 from STD) and Paddington-Stroud (16:30 from PAD) on Thursday afternoons (I stay in London Tues-Thurs).... so one stop further than Kemble and less 'peak' than what you're considering... and doing it one way is obviously less painful than doing it two ways on one day. But, all that aside, I find the service surprisingly reliable, although the London-bound train seems to be arriving at Stroud a minute later and later every week right now. So don't back on always arriving at PAD bang on time. Some other things to be aware of:
* The trains are getting noticeably busier week by week. They used to be nine-car trains but switched to five-car a few years back (either due to Covid or a timetable change - I forget which) but, either way, it may soon reach the point where they really should find a way to go back to nine-car.
* I've always been able to book a seat on the service I want - and I've always been able to book a seat on the day of travel if I've had a flexible ticket and wanted to get a different train.
* And booking a seat is definitely advisable... you do sometimes see people standing/sitting in the vestibules.
* Sometimes they forget to turn on the reservation lights and it becomes a free-for-all (or, worse, they announce the train at PAD but only turn on the reservations AFTER a bunch of people have boarded). But this could happen anywhere I guess
* Do you intend to work on the train? If so, check your network's coverage. I'm on three and get a perfectly good signal to tether from between Paddington and Swindon, but it's very patchy for significant sections between Swindon and Stroud, especially around Kemble.
* I know it's subjective but I've always found the journey to _feel_ as if it has passed very quickly. I wouldn't want to to the two way commute regularly but it's perfectly doable and, as you say, crossrail makes a big difference.
* I can often get a first class upgrade for my PAD-STD leg on seatfrog for £15 or less