Going to go through things as best I can while not trying to repeat what others have said.

1. I think lore would need changing I don't think they would want to make the crossovers fully canon to story could have odd nods to FFXIII in the ability descriptions but it would have to be grounded to Hydelyan. Fortunately there are plenty of places that a fire thematic healer(actually in love with this consept as fire has been used in mythology as a guiding light/great cleanser) could stem from like Thavnir/New World/Garlemald/Meracydia etc.

2. Needs a basic spamable single target heal and aoe one too could change Sun 1 and Sun 2 to these. Again I like the concept for Sun 2 and 3 being placed aoes that could pulse out for heals etc but the above must take the forefront as a lot of aoes happen in succession in ShB content (some previous expansions too).

3. Pyrotechnic chain could allow the spamable heals in it and this could be an oh no button if needed or for damage when not.

4. Are Geyser/Flames of Y/Stellar Canvas all sharing the same cd or can only have one up at a time? Because if can have all 3 up at once that is a lot of buffs to maintain in short timeframe. Especially when you add in every 3rd brighting radience and the grand finales and brightening night.

5. I suggest changing the get top enmity to something like a distraction debuff which lowers dmg received or prevents crit etc

6. While the fire dmg aspect is very flavorful, this would most likely be changed to magic dmg for easier to code with.