You can cast a spell on the same turn as using the starry form attack
As you've mentioned, the starry form attack only states that you must "make a ranged spell attack", but nowhere does it say that it is a spell. Regardless, it also doesn't state that it uses a spell slot, which is the real restriction on whether or not you could cast another ...
If the additional bonus action was casting a spell, it would say so. But it doesn't - it just says you can use it to make a ranged spell attack (meaning your use your spell attack modifier for the attack).
Therefore you are free to use your action to cast any spell you like (as long as it has a casting time of 1 action)!
Your action is restricted to a cantrip only when you actually cast a spell with a casting time of 1 bonus action:
A spell cast with a bonus action is especially swift. You must use a bonus action on your turn to cast the spell, provided that you haven't already taken a bonus action this turn. You can't cast another spell during the same turn, except for a ...
It depends on how you conceptualize damage
Answers to the following question show that the understanding of vulnerability and resistance is not a wholly agreed upon thing:
How does resistance/vulnerability/immunity interact with carryover damage after reducing Polymorphed (or Wild Shaped) form to 0 HP?
The rules on "Damage Resistance and Vulnerability ...
As far as I can tell, the damage taken is only calculated once. The same applies to a spell-caster with stoneskin up who loses concentration--they gain the benefit of the resistance for the triggering damage, but lose it for future ones.
The order of operations goes
Calculate raw damage (the result of the damage roll)
Apply resistance, immunity, and ...
Let's look at swarms for a second shall we?
A swarm is immune to any spell or effect that targets a specific number of creatures (including single-target spells such as disintegrate), with the exception of mind-affecting effects (charms, compulsions, morale effects, patterns, and phantasms) if the swarm has an Intelligence score and a hive mind. A swarm ...
The Kalashtar features come from a spiritual link; Wild Shape does not sever this link
The Wild Shape feature states:
[...] You retain the benefit of any features from your class, race, or other source and can use them if the new form is physically capable of doing so. [...]
So we only need to know whether a given new form is physically capable of using ...
All four of the racial features mentioned are usable while in a Wild Shape form
The pertinent part of the Wild Shape feature description for this question is as follows, which is part of one of its bullet points.
You retain the benefit of any features from your class, race, or other source and can use them if the new form is physically capable of doing so.