Funnily enough I have just been looking at this for another forum member (off line), and going round in ever decreasing circles.
At the moment we don't know. Currently Cross Country (who run part of the overall service between Plymouth and Bristol) only have timings in the system for Sundays until 17 August, on that route (later on other routes).
This is the first day of a week of engineering on the Taunton to Bristol direct line
However they are suggesting that they will run diverted trains via an alternative route (which can only be via Westbury and Bath, between Taunton and Bristol) which will apparently add up to 1 hour to journey times.
Whether you would get advance fares on those trains, or have to pay full fare is an unknown quantity. But if it adds an hour then it will take as long as the Great Western bus transfer from Weston, maybe longer.
You can either wait several weeks for Cross Country to produce their plans (and probably achieve nothing) or bite the bullet and book the Great Western via Weston, then bus to Bristol, then train forward from Bristol.
If you took the Cross Country diversion route you would have to change additionally at Plymouth from GWR onto XC.
Great Western are NOT diverting any trains so what you see is what you'll have- at least there are advance fares on that route.
Yes, unfortunately, whatever you do you will arrive later than planned.
Just to be complete there is a National Express Coach at 0830 from St Erth, change at Bristol Bus and Coach Station, arrives Cardiff at 1805. It goes on a scenic tour of Cornwall taking 4 hours to reach Plymouth, hence the long journey time, also an overnight coach at 2214, arrives Cardiff at 0745 next day.