It isn't currently possible to spread the black dylium to other end stone blocks using bonemeal on the connected end stone block, the dark amaranth roots spread on all different blocks when bonemealing the black dylium block and the dark amaranth tree's leaves doesn't drop the fungus or bonemealing the black dylium block so the dark amaranth fungus can only be gained from the ones already generated in the end.

Dark amaranth fungus is indeed renewable since you can create an infinite amount from bonemealing black dylium