I may have found a way to get a lot more EXP for the entire party during the Erosian Prelude. I notified Sierra about it and she said it wasn't necessarily a problem but would take a note for it, so it may or may not be patched later on.
The roaming tower mobs that spawn in the Tak'kan tower stay even after the Prelude is finished, so you can avoid doing them until you have the entire party + Simon to take them down.
In the Zirantia tower section those roaming tower mobs remain as well until after the Prelude, with the exception of the top right roaming one, which you do need to kill. The static flame in the bottom left (which spawns a solidified chaos) can also be killed after the Erosian Prelude so Simon + the entire party can get its EXP.
In the Aram tower, while the roaming tower mobs should be killed ASAP, the two roaming Erosian mobs in the top part of the tower and the static one in the middle all remain as well, which means that in total Simon can get an extra ~ 43k exp.
But like I said I don't know if it will be patched so take it with a grain of salt if it's actually possible later on.