It depends on the campaign, for the "Is this balanced?" question.
I will focus on that in my last section, comparing your race to similar other races.
It's hard to say if features like Flight will be balanced without playtesting it. But, as can be seen in this question, the major problems dwell in early levels. While essentially granting a slower Fly spell for free (opposite to actually casting it, which requires preparing/knowing it and spending a 3rd level spell slot, besides requiring concentration) might seem too strong, it actually depends on how much opportunity you give for the character to use/abuse it.
Most published adventures for Tier 1 (levels 1-4) were (probably) printed assuming the party won't have any ways of flying, thus the flying races are not allowed in AL. From 5th level onwards, the creatures should have ways to deal with flight, as they could be facing Spellcasters with Fly as well.
From that, I would say yes, giving the permanent flight only at 5th level should solve the problems from original Winged Tiefling. How "balanced" it will be, though, still depends on the campaign. As I mentioned, Fly is limited by concentration and spell slots. A 5th level Wizard can only cast Fly twice a day. If there are 8 encounters in the adventuring day where flight helps a lot (i.e., where usually the Wizard would be casting Fly), the feature is still way more powerful than any other in the game. On the other hand, if your encounters are inside a dungeon with a 10ft high ceiling, the flight is worse than having a free cantrip.
From my experience, adventures will have a decent mix between encounters that easily hit flying creatures and encounters that flying will read "This creature is immune to any damage this encounter". In that case, the feature shouldn't be unbalancing/game-breaking. Note that it will still shine more than most other race features.
Note on ASI
Awkwardly enough, I think the major "problem" with your changes is the change in the ASI. The +1 Int for Tieflings, which appears in both the usual and the feral variant, usually meant that the Tiefling only had the +2 primary stat, as Int is a shadowed stat, only used for Wizards. By changing that to Dex, you are giving a free +1 "actual" stat for classes like Sorcerer and Bard, which are probably very happy to get Tiefling now.
Comparing to other races
The comparisons, IMHO, are now different than the ones I would usually make for the usual Tiefling (or even the Feral one), due to the ASI changes.
I will focus my comparison with races that have a similar ASI to yours, i.e., Half-elf (+2 Cha, +1/+1 chosen) and Drow (+2 Dex, +1 Cha), since these are the other common races that a Bard, Spellcaster or Warlock would be getting if they weren't choosing your Tiefling, from my perspective. For me, it doesn't make sense to compare between your race and, for example, Dwarf, as I can't see someone actually having to choose between them from an optimization POV.
Comparing it to Half-elfs, you are now trading 2 skill proficiencies, +1 ASI, Fey Ancestry and an Extra Language for resistance against fire and flight at 3rd/5th level. Clearly, your race has better combat applications, exception being for fights against (control) spellcasters (where being immune to sleep and advantage against being charmed are stronger than flying, being paralyzed by Hold Person and taking fall damage). On the other hand, social adventures will favor the Half-elf.
Comparing it to Drows, you are trading Proficiency in Perception, Fey Ancestry, +60 ft in Darkvision and Drow Magic for Fire Resistance, your Flight and not having Sunlight Sensitivity. Again, I consider Fire Resistance more useful than Fey Ancestry, generally, but that might be biased by my adventures experiences (fire damage being more common than charming effects and/or sleep inducing effects). Other than that, the choice becomes essentially "are we fighting outside or in a dungeon?" - outside your race is clearly superior, inside the Drow is clearly superior.
From these comparisons, I say that your race is balanced, as it is not a clear choice over other similar races in every scenario (which is how I read "overpowered"). It might present a problem with very particular scenarios where flight will make that character shine too much, so, as a DM, I would just avoid creating such scenarios, unless specifically for that player and PC feel good about their choice and get some spotlight.