So it ocurred to me: At no point did Galliard stated that he became a Titan Shifter after eating Ymir, only that he had recovered the power of "his brother's jaw". What if he was a titan shifter before Ymir's death (maybe the claws were his trait) and he ate Ymir, uniting the 8th and 9th titan? That would explain why we haven't seen the 9th titan neither in Marley nor in Paradis: they were the 9th and 8th titan shifter, now combined into one, reducing the number of shifters to 7.
I know there are some possible holes in this theory:
1. "Why, if Galliard was a shifter, wasn't he send to recover the Founding?" maybe he wasn't. maybe he inherited the trait after the mission was send, from its previous owner. Zeke wasn't send from the start, for example.
2. "Why would Marley allow a single titan shifter to become more powerful?" Maybe for that same reason. Knowing that the power of the titans was being challenged, maybe they aimed to create a stronger titan by fusing both lines, as they appear to be compatible.
Would like to hear some thoughts on the matter.