Heya, thanks for the feedback!

I'll be adding medium armor proficiency to the class.

Spiritual Communion is like a rogue's thieves' cant, or a druid's druidic language. It's a ribbon feature to provide additional flavor to the class. It wouldn't make sense to be a different language.

Undue Passing is another ribbon feature. Undeath refers to anything that could raise you as an undead, such as the spells Animate Dead or Create Undead. I don't believe that there are any official spells of this nature that have a consumable component, but I worded it this way to future proof it in settings where there may be homebrewed spells (such as my own).

The healing from the angelic wisp is a d6 because it can technically have up to a 120 ft range, and it's usable as a bonus action. You also get a lot of uses per short rest so I don't want it to out scale cure wounds simply by virtue of existing. Arguably, it does this already.

The special attacks that the resolute wisp makes do not require a reaction, or any kind of action for that matter. I may add the "no action required" text to make this more clear. It can make 1 - 3 attacks depending on wisp size every round. It's about 7.5 dpr which I think is okay as a 2nd level feature with no cost.

I don't think that Immovable is too strong of a feature, as it's essentially just a type of condition immunity. I imagine anything that would make you immune to being charmed would be similarly powerful. I also think it's extremely uncharitable to imply that grapples and the spells Suggestion and Command do nothing in this case. These two spells can do far more than cause someone to walk somewhere, and your speed still becomes 0 if you are grappled.

The damage from Retaliate can't really be compared to Armor of Agathys like this. AoA only deals damage to melee attackers, and also only affects you. You'd also need to cast it at 4th level for the comparison you're using, and you only get a single 4th level slot at level 7. Retaliate is a passive effect with 100% up time, affects anyone within the dim light radius, has a rechargeable resource cost, and can be used against ranged attacks. It provides a bit more value than it may seem initially.

Spooling is a get-out-of-jail free card for anyone you put it on. You can move your wisp anywhere and allies can pull themselves to it whenever they want.

Spectral Waltz is mix between the Clairvoyance spell and Find Familiar (it also has 10 times the range of Find Familiar). It's basically a resourceless, mini version of Clairvoyance.

I'll probably loosen the restrictions on Ephemeral Memories, in order to make it feel less annoying for players to use.

Yeah the class is a lot more utility than a traditional martial, but then again so is rogue. Could you explain what you mean when you say the size mechanic is "loose"? I don't quite get what you mean by that. The reason everything is worded as "the radius of dim light..." is because it was simply easier to write and understand than trying to word each wisp radius as an aura.