Wide variety of naval mines that would make it much more difficult for China to invade Taiwan
A relatively low cost way to make the invasion of Taiwan by China much more difficult and time consuming would be having the capability to rapidly flood the Taiwan Strait with mines.
There are many types of mines that can be deployed, but influence mines would be the most common. These mines come with a wide variety of capabilities including being able to be programmed to avoid obvious mine sweeping efforts and only attacking ships with specific acoustic and or magnetic signatures. Other mine types that could be used include drifting mines, mobile mines, subsurface drifting mines, etc.
As noted by Scott Savitz in his excellent Proceedings piece the exact mix and deployment locations of mines would be key to their effectiveness. But used and deployed properly they would deter China by:
Extending/protracting: China's timeline would be lengthened as no way would China be able to move it forces by sea as quickly in in the face of extensive mining.
Disrupting Coordination: To the degree that some of China's military assets are more vulnerable to mines than others and that it will be very difficult to predict just how much any naval asset will be delayed due to having to deal with mines, China's ability to coordinates attacks will be disrupted.
Channeling traffic: In dealing with mines it is standard practice to clear mines out to allow a path for ships to transit. This effectively channelizes the ships transiting a mine field and makes them vulnerable to other attack as they become easier to track land attack as their freedom of action/navigation is severely reduced.
Creating obstacles:. Ship sunk in shallow waters by mines created hazards/impediments for ships trying to conduct operations near landing zones. Damaged ships increase risks of collision risks if their ability to maneuver/steer is damaged.
Impeding use of operating areas: Establishing standard areas of operatoins to facilitate landing troops become very dangerous as the longer ships are in any particular area, especially areas in vicinity of optimal landing sites, the more likely they are to strike or be struck by a mine.
Along with mines, helping Taiwan expand its almost non-existent submarine force with modern AIP diesel electric submarines would be the most cost effective deterrents to any attempt by China at a mass invasion by sea.
FOR MORE DETAILS ON USING OF MINE TO DETER CHINA
For a more detailed view on the use of mines to protect Taiwan see Scott Savitz's excellent article in the February 2023 edition of Proceedings Magazine.