I think the 075 is probably more targeted for a SCS scenario than a taiwan scenario which might explain the small number built
In the case of taiwan it's probably more cost effective to build smaller landing ships and rely on ground based helicopters
For Taiwan, it's almost certainly more cost effective to buy the [Type-072A LST] over the [Type-075 LHD]
I reckon a Type-075 is easily $800+ Million
And a Type-072A is about $14 Million, based on the comparable cost of the Teluk Bintuni LST.
So 20 Type-072A would only cost one-third of a single Type-0975, but would have roughly twice the cargo capacity.