Shinryu is the final boss of Stormblood, that is fact. The way the story is told means that the liberation of ala mhigo ended in 4.0. This is different from HW where the dragonsong war ended in 3.3, as the main villain, Nidhogg, was not dealt with until then. Vishap also was not the final boss of ARR, Gaius and Ultima Weapon were, Vishap was the prologue to HW.
You are right that there will be a continuation of the story, probably involving fordola and yotsuyu, but the final boss is Zenos/Shinryu