The problem with building a LHD vs. another LPD is that there's no real reason why the ROCN needs to deploy more helicopters to the Pratas and Spratlys than a LPD can carry (regardless of how much more a LHD costs, the extra helicopters will cost a lot more).As the saying goes, steel is cheap. It probably isn't that more expensive for Taiwan to built something that size (I think it's only 10,000 t?) than something a bit smaller.
It's probably also even useful in peacetime, as last year China played silly buggers by closing HK's airspace to a Taiwanese transport flying into the SCS "for reasons". Having more options to resupply outlying islands is therefore a good idea.