One of the last (few) seems right to me
You'll never remove the
last bottleneck for as soon as you do, somewhere else will become your capacity limiter.
Now that Reading isn't the bottleneck on the
GW▸ main line, Didcot is. Fix that and you're looking at Royal Wootton Bassett ... then Bathampton, Swindon (Kemble line), Thingley and Didcot (West). You're going to need extra platforms at Paddington, Didcot, Chippenham and Bath Spa to be off the main line ...
There's no bottleneck only when you have more capacity than requirement!