Tips on playing a Merchant Republic - post 1.103 patch

Hello folks,

I just picked up the Republic expansion from the summer sale and I'm a bit lost on the mechanics surrounding merchants (assuming ToG start date).

AFAIK, trade post limit is now limited, so my only way to expand trading zones is through war declaration on other republics.

Upgrading the family palace is important, ok, but which upgrades are more worth it?

Can merchant doge's become kings? Is this advisable?