The wildest idea is to totally disconnect ports from inrastructure. If you look at ports, they are only the place where goods are swapped between land and sea and vice versa. There is 0 logic for them to provide the state infrastructure, if anything they should consume it. Ports need vast networks of roads, trains and inland canals to supply the goods inland. If they are attached to a big city and/or urban state, they of course use this infrastructure for their own benefit. Think about modern ports. How do we get todays containers to the cities where consumption is happening? They always have a very good highway connection, rails or loading places for canal and river transportation. In Victoria 3, these assets are separate modelled, so infrastructure is not part of the ports.
I also do not understand why we still have convoys. We now have merchant marine (which we could simply name trade marine), which is more then enough in my view to do what we need: The country can also buy merchant marine to ship goods for treaties. I mean they already do it for goods in treaties, why not for other government tasks as well? They can do the same for port connections, let the market decide when ports are in need to expand when the prices start to rise with demand. If the government wants o keep the prices low, subsidies can still be applied. Use clever auto-expand rules to ensure the prices stay low. Shipping goods within internal market areas is not in need to be done from the government, the private industry can handle it.
It is already tracked how many trade certain nations ship via sea nodes. Convoy raiding is currently still based on convoys, why is that? I still remember the system Empire: Total War used: They had a global shipping lane system as well. If you placed a fleet with raiding on the lane, the value the factions you were at war with were reduced, depending in the volume and your fleet size, and parts of that was added to your treasury. Can we not also do something like that? This would also maybe finally give us a situation where fleets are needed to patrol shipping routes to ensure they are free when at war.