There are only a few beaches on the eastern side of Taiwan suitable for mass landings, and they are well prepared kill zones covered by artillery, mines, tanks, and other obstacles.
They could pound the island with missiles that may or may not get by the PAC-3s we sold them. They could threaten to or actually use nuclear weapons against Taiwan, but that would destroy the very thing they want: the riches, expertise, and infrastructure of the island.
Plus, they face the very real risk of being badly humiliated by a very tiny opponent. The Chinese Army made it all of 20 miles into Vietnam in '79 before being torn to shreds and sent packing. Many of the chronic weaknesses that were present then are present now.
Does that mean they won't try? Who knows. Miscalculation and overconfidence are the seeds of many a military disaster.