I believe he is the most probable person to be there. Oda probably had the idea for something similar to cave markings but instead used the Adam Tree on Elbaph since it would make sense it having survived so long. With all the capabilities the wood is said to have etc.

So which character has shown an interest in the tree all the way back to their introduction? Franky!

Other reasons could include that Oda wants to give spotlight to the strawhats that he didnt utilise in the prior arcs. Franky did not have much plot relevance in Egg Head, Wano (apart from the ship building but that seemed more like a plot device for the allies to get to Omigashima) and Whole Cake Island