I have always thought it was odd that Thayet and Jonathan had so many children. The whole situation seems ripe for a civil war or rebellion several generations down the line! Sure, maybe they were all needed for alliances, or the royal couple wanted plenty of spares after the precariousness of Jonathan being an only child, but I have another theory. What if Thayet wanted one child for the throne of Sarain?
Thayet is a child of both ethnic/social groups of Sarain: the lowlanders and the K'miri. She flees Sarain in fear of her life and comes to Tortall, where she and Jonathan form an affection. Jonathan realizes that she is an appropriate and worthy royal partner; Thayet concludes that being the Tortallan queen is a role she can perform well and will be a meaningful use of her talents.
However, I refuse to believe that someone as loyal and with such a strong sense of duty as Thayet would abandon her people and wholly devote herself to Tortall. By the Wild Magic series we see that Tortall (no doubt due to Thayet's influence) has opened its doors to K'miri refugees. I think it is entirely possible that Thayet would have made one of her conditions for marrying Jon and sacrificing any hope of returning to Sarain that their children would be raised with an awareness of their mother's heritage and that one would ultimately be sent to Sarain to consolidate power and end the brutal civil war.
I don't mean that Tortall would send a conquering army to Sarain. There is a great deal the Tortallan throne can do without supplying an army. They can provide money for mercenaries, train any K'miri refugees in Tortall who want to return to their homeland, arrange diplomatic connections in the other Eastern lands, back loans made by banks to the prospective Saren government, send knights and other expert advisors, etc. I would think that the countries bordering Sarain would also have a vested interest in ending the war due to the spillover of violence and refugees into those countries and would provide assistance.
In addition, this is not a case of Tortall conquering Sarain! Thayet is twice-over of royal blood; her parent's union was intended to produce a royal that both the lowlanders and the K'miri could accept. (In fact, much like Dove in the Copper Isles. A parallel...?) She is a rightful claimant to the Saren throne, and so is her child.
This would also have the effect of keeping one of the Tortallan royal children (let's be real, probably one of the princes) busy and away from any resentment or rebellious tendencies. Once a stable government is formed in Sarain, there could be strong diplomatic ties between the two countries. There are conveniently several countries between Tortall and Sarain, which makes it less likely that a future Sarain would attack Tortall to claim that throne.
Anyway, this is my explanation for Thayet's agreement to be the Tortallan queen despite her Saren loyalties and one reason for their six royal children. I haven't read Lioness Rampant in many years, so I might be misremembering some of the details. I also haven't kept up with Word of God or extracanon stuff. But I find this headcanon satisfying lol.