Honestly I think this would be easily achievable!
For example, even just two facilities, Pairi Daiza and Hannover, (founding with Manari, Sayang, and perhaps 2-3 imported bulls and another 1-2 imported cows), could easily form a strong base for a pure Bornean population, and between the two of them alone, could hold a good 30-40 animals of assorted ages.
For Sumatrans, they could turn Spain into the pure Sumatran region. Along with Madrid, Benidorm, Estepona, and Sevilla could all join into pure subspecies groups after sending their current animals (all nonbreeding older cows and a single sumatran x mainland bull) to other facilities. It could even be an easy, gradual change. Import another bull or two, and another cow or two. Send the two old cows from Estepona to Sevilla to make room for the new group. As Madrid and Estepona breed and grow, eventually a cow group can be split and sent to Benidorm. Sevilla can serve as a massive bachelor repository. They can also hold the old non-breeding cow group, considering its a bunch of 40-50 year old cows, it'll likely dwindle off by the time they really need the space for the bachelor bulls produced by the new groups.