Then I go to the diplomancy screen and do a trade embargo on all remaining AI.
End of quote
That is so totally different from the type of tactics I would use... I would have thought the AIs could easily make plenty of money from trading with each other. My main concern is making sure I have a reliable income. As long as I have money coming in, I can buy and bribe my way out of any problems. So I do whatever I can to encourage AIs to trade with me, such as colonising as many worlds as possible in order to attract freighters, giving away trade route technologies, and getting AIs to go to war with each other in order to break up their mutual trade.
I don't see anything dishonest about trading PQ14 worlds to minors. Unless you're low on money, PQ14s are very useful, with a bit of development. A source of influence, ship building, and technological research. The only issue is why AIs with planet quality bonuses don't bother colonising them themselves.