Something that might be worth investigating is converting the end of the branch to OTWS (one train without staff) working, using an axle counter, in place of the key token. Even though full 'track circuiting' would thus be provided all the way to the buffer stop of the single line spur, classification as OT instead of TCB would avoid the requirement for a signal for reversal at the terminus so the axle counter would only need to be single-ended, with sensor equipment solely at Tondu and no need for any new signalling equipment beyond there. This would avoid having to stop for the token exchange in both directions outside the signal box, perhaps saving a minute or so in each direction. The token exchange might alternatively be relocated to the station platform at Tondu with a remote instrument for train crew to use in a hut, Central Wales style, to avoid the double stop, but this probably wouldn't save as much time as an OTWS conversion.
For further resilience, I'd also definitely look at a short bit of double track through the main line junction at Bridgend and for around one kilometer along the branch, to just short of Wildmill, thus avoiding any expensive station reconstruction. Something like the following:
