This "any permitted route" clause is complicated, as none of the options I've looked at show the route via Birmingham. One was even via Droitwich & Bromsgrove.
I found the ^12 (railcard) Anytime Day Return,
any permitted Cradley Heath to Ashchurch by putting Moor Street as a via point in to a Journey Planner. It was then a case of looking for suitable services that would get you into Brum and from Brum to Taunton.
I've been careful with more than one ticket for "one train" journeys in the past, as I thought the train had to stop at the stations referred to by the tickets. You live & learn.
With normal split tickets, say two Off Peak Day Returns, it is indeed the case that a through train must call at the split point. But if one of your tickets is a Season or a Rover, just passing through the split point is acceptable. I'm sure there's a reason why this is so, but I can't think of one!
Wow, this does get complicated!
Your not wrong, it helps if you are borderline OCD like me when it comes to finding the best/cheapest/most interesting way to make a journey. And if it's any consolation, there are no split ticket opportunites between Bishop's Lydeard and Minehead!