I might agree with you on the flight speed or dexterity, though by 5th level you do get access to the Fly spell which does the same flight speed. The 60ft flight speed on Radiant Wings would only last for a minute.
Yes, but being able to access the fly spell is different than granting an ability that is similar. For one, Fly requires resource expenditure of a 3rd level spell slot, which is quite valuable, and also requires concentration. Thus the Fly spell limits your ability to cast other concentration spells, and can be negated if you take damage and can't pass a Concentration check. None of these limits apply to the flight granted by your feat.
Your table may have a different balance point. But if the goal is to create something that works with existing mechanics, we need to look at what is granted by similar feats or abilities. And no feat that grants flight grants the ability to fly at such high speed, or grants more than
maybe a point increase for an ability score.
As for the reason behind not barring Heavy Armor, Urd Wings are always on as are the Teifling variant wings, where Radiant Wings needs an action to activate. In combat that makes a difference, also Sleep and Unconscious effects would necessitate activating the wings again. The former Kobold and Teifling are getting a completely new ability, while the latter Aasimar already has said ability this is just an upgrade. That was my reasoning.
Thanks for the feedback.
Sorry, but I don't buy this argument, and I have a few reasons.
1) Yes, the flight granted by your feat requires an action to activate. But there is no limit or duration for that activation. A character with this feat could wake up, activate their flight, and maintain them all day. This there is no need to bring action economy into the equation because you are likely to start combat with your flight already active.
2) We can look to another similar ability. The Dragon Sorcerer gains a flight ability. This ability comes online at level 14. It limits your fly speed to be equal to your walking speed. It must be "activated" like the flight in your ability. However, a character needs special armor and clothes to accomodate these wings. It does not have the same limits regarding heavy armor or encumberance level. But also, it is a high level ability.
So your feat allows a 4th level character to achieve an ability on par to a 14th level character, but does not require special accomodations for clothing and armor, and grants half an ASI, allows you to activate a racial ability more quickly than normal, and gives a temporary speed boost to double your normal movement.
So perhaps this feat may work at your table. And no, I don't think it will break the game. But given existing feats and abilities, I do not find it to be balanced.