At level 10 the new Circle of Stars Druid can fly while in their Dragon Form. There is nothing as written that says they can't polymorph and maintain their Starry Form. So flying T-Rexs that also get a buff to maintaining concentration? 

There's an interpretation issue that I would have an issue with, from Polymorph:

The target's game statistics, including mental ability scores, are replaced by the statistics of the chosen beast. It retains its alignment and personality.

This, for me removes Starry Form, but more importantly, later it states:

The creature is limited in the actions it can perform by the nature of its new form, and it can't speak, cast spells, or take any other action that requires hands or speech.

The target's gear melds into the new form. The creature can't activate, use, wield, or otherwise benefit from any of its equipment.

I'd rule that that means even if the Starry Form is maintained in the manner you can't use it to fly.

Polymorph - no.

Wildshape - yes.

Wildshape - no

As a bonus action, you can expend a use of your Wild Shape feature to take on a starry form, rather than transforming into a beast.

Wildshape usage into Wildshape usage removes the previous features. In the same way as you don't keep features if you wildshape directly from an owl into a bear.

Quote from BurnishedBronze >>

Wildshape - no

As a bonus action, you can expend a use of your Wild Shape feature to take on a starry form, rather than transforming into a beast.

Wildshape usage into Wildshape usage removes the previous features. In the same way as you don't keep features if you wildshape directly from an owl into a bear.

The reason why Wild shape from owl to bear doesn't keep the features of an owl is because you can only be effected by 1 effect of the same name at a time.

Combining Game Effects

Different game features can affect a target at the same time. But when two or more game features have the same name, only the effects of one of them—the most potent one—apply while the durations of the effects overlap. [...] Game features include spells, class features, feats, racial traits, monster abilities, and magic items.

Since starry form and wild shape have different names so they can both apply unless a different specific reason is given (such as how polymorph causes all statistics to get replaced).

Quote from Lyxen >>

Quote from DxJxC >>

Since starry form and wild shape have different names so they can both apply unless a different specific reason is given (such as how polymorph causes all statistics to get replaced).

 
This is a good call, and the first part of the Starry Form supports this as it is only saying that it is expending one use of the Wild Shape. Unfortunately, the end of the sentence makes it less clear for me: "As a bonus action, you can expend a use of your Wild Shape feature to take on a starry form, rather than transforming into a beast."

The end of the sentence makes it look like it is actually a Wild Shape itself which is being used, but in a different shape.

Personally, I don't like this "transformation on transformation" principle, 5e operates on the basis that everything is transformed into the new shape to avoir overlapping templates likein 3e, which were really a mess, so I would not allow both at the same time, but YCMV.

 

 
the rather than, to me, seems no different than divine smite uses a spell slot rather than casting a spell.

that said.... divine smite doesn’t specifically use the phrase “rather than”