What determines Nanban ports to be built?

[Warhammer3025]

Hello there, first post for this game! Longtime TW veteran here with a question. I may have found a bug or something's thats WAD but want confirmation. Playing as Shimazu atm and going full on christianity for the guns and the religious challenge. Built a Nanban port in my home province and conquered the northern Shoni who converted before me and also had a nice juicy Nanban port of their own. However as i was teching up my other province harbors they wont let me build any more Nanban ports, just military ports. Now Nanban ports give lots of money so i wonder if i'm limited to being able to build em one time, thus getting lucky with the Shoni port, or did i hit a bug?

Also, is it me or do all clans seem to be forced to rush hardcore ashis in early game (playing VH/VH)? My first playthrough with Oda, i naturally went Ashi swarm and had a ball overrunning folks, hell i didnt even bring archers to the party. With Shimazu if i dont build up he human swarm i simply cant compete with the nearby clans thus forcing me to bust out the ashi swarm again (although the superior katana inf is nice). Similar thing with my 2nd playthough with Hojo but i figured that an ashi swarm was reasonably needed since i'm sandwiched tween 3 strong clans in the Hojo, Uesugi, and Date (sad how the Imagawa got demoted to minor status). Also how come the major clans are so sadly weak, every game the buggers keep getting wiped out within 10 turns.

C'mon now, as Shimazu i wanna face a Hojo/Uesugi/Takeda swarm ahhh the good old days of shogun....

Edit.Lets watch the language a bit. - Kagemusha