Light ships seem to be a merchant marine type of system, rather than a formal navy or purely mercantile fleet. The idea is that they are inferior for both cost and force limit in naval combat, but are not defenseless vessels. It also fits in line with how protect trade currently works, where you effectively patrol the home waters of an important node. Realistically, heavy ships or galleys should be the ideal bulk of a fleet, not light ships.
The idea of using light ships to protect the home waters works when you think of it as routing out piracy and especially smuggling. It fails when you factor in that this is the bulk of your trade power and even worse, propagates downstream, which is something that dedicating heavy ships towards convoy duty would far better simulate. You also really should not be able to protect more trade via ships than you generate from trade power. Having a protect trade (light ships, cut down smugglers) with a firm cap that it cannot exceed the total production of the node itself, and possibly a support trade (via heavies) that lets you pull and steer trade, makes more sense. Really I just despise that trade power comes from light ships, even far overseas, and even in zones you don't have a port in.
Trade just needs a huge overhaul I guess.
This is just pipedreams, but I think a good system might be to have three different types of "Trade Missions", with different types of ships being good at (or even able to perform) different missions, and unlike the current system they are
not mutually exclusive unlike the current privateer and trade missions.
The three missions would be Treasure Fleets, Privateering and Trading, each with their own purpose:
Treasure Fleets ensure that trade value contributed by your own provinces (as well as those of your colonies and vassals) get to the destination you want, and propagated downstream; if Spain has the entirety of South America as Colonial Nations and Vassals, then setting fleets to perform "Treasure Missions" in the Caribbean tries to ensure that
all trade value they contribute to the node gets sent onwards to Sevilla. Basically instead of increasing your own trade power in a node, treasure fleets decrease the non-provincial trade power of everyone else, and channels the provincial trade power of your subjects to your own ends. Treasure Fleets would
also be able to fight Privateer fleets in the same node without declaring war, and would attempt to hunt down all weaker Privateer Fleets.
Privateering would function like it does currently, only buffed. Because Privateering should be a legitimate way of enriching your realm, not merely a way to acquire Power Projection and ruin it with bad events. Privateer Fleets would also be able to fight Treasure Fleets without declaring war, and would attempt to hunt down weaker Treasure Fleets, but also to escape stronger ones.
Trading would contribute to the wealth of a node, and would function a bit like trading currently does. Trading Fleets can contribute trade power in nodes where you have none, and can increase the both the power and value your merchant adds to a node.
Like I said before, different ships would changed to help them perform different missions:
Heavy Ships can only do Treasure duty, but are the ships contributing the most trade power to this mission. Each Heavy Ship in a Treasure Fleet would also contribute a greater percentage of their own trade power to downstream nodes than other types of ships.
Light Ships can perform both Treasure Duty and Privateering, but not Trading. They aren't as good as Heavy Ships at Treasure Duty (even taking maintenance into account), but due to their speed would be much better at actually hunting down privateers.
Galleys can do the same as Light Ships, and have the same strengths. In addition, Trade Ships also get a speed boost to be as fast as Light Ships in Inland Seas.
Transport Ships (when not transporting troops) are the only ships able to perform
actual Trading. They have the hull space and privately owned merchant ships where often repurposed as transport ships in war (notice how every Transport Ship in EU4 was a type of trading ship in real life, Fluyt and Cogs for instance).
In addition, NIs would be changed so the Hanseatic countries get their light ship bonuses replaced with more powerful bonuses to Transport ships, while Dutch countries have their Light Ship NIs reworked to both give the same bonuses to Transport ships and smaller bonuses to Light Ships.