Overseas resource strength, supply, pockets

[The Land]

So I have a suggestion that could probably be made to work about improving overseas supply.

Currently, I feel it's too easy to get supply into harbours. Blockading is difficult and risky (as it uses the 2-units-adjacent rule that also applies to land sieges), and you can have totally enemy controlled seas with supplies somehow still reaching a harbour even if there is nowhere they can have set sail from.

So my proposal is:

*Harbours should only be able to give supply if they can trace a path over sea hexes (or friendly harbour/straits/canal) to a friendly harbour that's connected to the capital, and which itself has high enough Strength. Special treatment would be needed for the UK and USA in the Red Sea, Persian Gulf any maybe South Atlantic. (Possibly there should be a limit to the distance this path could be as well).
*Harbours should risk taking a temporary strength hit if there are enemy units within a few hexes (maybe 3-5). The probability and size of the hit should depend on the balance of naval units in that area e.g. if there are a crowd of enemy ships the blockade should be very likely, if there is one sub faced by 3 friendly destroyers then it should be slim.
*Overseas resources should be limited to strength 3 if there is no good path back through an unblockaded port back to an unblockaded friendly harbour - ie they should all be pockets.
*Probably there should also be a limit on how many road hexes this kind of overseas supply should be allowed to cross - maybe depending on logistics tech? It strikes me as a bit odd that e.g. the Axis can ship so much oil from Iraq along that one road into Palestine... However it'd probably be best for this limit to only apply to mines and oil wells (to avoid messing up the supply system even further)

What do people reckon...? To my mind this would result in a much stronger role for navies!