Can Est be a good defensive flier ?

Est has always been a good mage killer with Distant Counter. Berkut's Lance ++ would make her better at her job, but...

She's still good.

It's a great build. Only problem, if you have all those resources to spare, is that blue isn't the BEST color for a res tank. Red mages are the least common iirc, so you'd probably want to run DM3 to guarantee she can take Rein.

Camilla uses that build better, as being able to devour blue mages and most greens as well is super valuable for arena. I've had good experience with red anti-mage units, as green is the second most threatening magic color, with Nino, Blade Cecilia, and some Sonya/Julia/Deirdre being big arena threats as well. But that Est will be amazing, especially if you really just want to build her. Will have 0 trouble dispatching mages whom she can survive the first round of attacks from with her attack, res & QR3.