
My problem wasn’t with the Druid list, it was that they only get about half the Druid list. This potentially means freely maintaining a concentration-duration spell indefinitely.

This is a large part of the Synad's main shtick, and while he gets to do it once per day (although with more flexibility) you get to do it ALL THE TIME. The fact that you can maintain concentration while doing other things and even while actively opposed is a powerful ability you should try to make the absolute most out of. It is actually the only other ability they get that cannot simply be replicated by a spell. The other major ability they get that does not concern 'spirits' is Guide Magic. This is why I kind of like theurging with warlock, quickly sidestepping for a few levels into Enlightened Spirit, to emphasise this trait - probably picking permanent speak with animals along the way.

Most of the other abilities the SpSh gets are designed to make them the party resident expert at fighting incorporeal creatures. A dip into a Prestige Paladin/Bard or Sand Shaper can extend this list a bit (the latter in particular to allow for some more fanciful direct spells), but most large-scale list additions really give you more of what you already have (which is why they are great for arcane classes that usually get less of that). This by default makes them limited to indirect spells such as summoning, healing or buffing. That said, they pull from arguably one of the weaker spell lists in the game: the Druid list. Depending on your DM, Chastise Spirits may or may not qualify you for things that require Turn Undead. Therefore, if you are going to use a spell at least twice it is always worth retrieving for the day.

The amount of spells so gotten never becomes less than twice your daily casting allotment, and is usually more. This makes them 'spontaneous' to a degree, while still maintaining access to their whole list (allowing for the same list-growth shenanigans that arcane fixed-list casters get up to).
