It's never made much sense that the entire multi-species Federation would be subject to a strict ban on genetic engineering due to events on Earth that happened centuries before the Federation was even founded. The way they doubled down on that rationale in Una's trial only highlighted the absurdity -- especially when Admiral April claimed he would exclude Una to prevent genocide.
On the one hand, the writers may be trying to create a straw man out of a weird part of Star Trek lore so they can have a civil rights issue in Starfleet. And that's fine. From an in-universe perspective, though, I think we can discern another reason for the ban on genetic engineering -- the Klingon Augment Virus.
There was a ban on genetic engineering on United Earth, which is understandable given that it was much closer to the time of the Eugenics Wars. Why would that remain unchanged when more time passed, more species joined, and more humans lived in places without living reminders of the war? [NOTE: I have updated the paragraph up to this point to reflect @Value Subtracted's correction in comments.] The answer is presumably that they needed to reassure the Klingons that something like the Augment Virus would never happen again. Hence they instituted a blanket ban around that time -- perhaps in 2155, the year after the Klingon Augment Virus crisis and also, according to Michael Burnham, the year the Geneva Protocols on Biological Weapons were updated.
That bought the Federation over a century of peace, but after war broke out due to a paranoid faction of Klingons who thought humans would dilute Klingon purity and after peace was only secured through the most improbable means, they doubled down on the ban. Una's revelation provided a perfect opportunity to signal to the Klingons that they were serious about the ban -- hence why they would add the charges of sedition, perhaps. Ultimately, an infinitely long speech and the prospect of losing one of their best captains combined to make them find a loophole -- but not to invalidate the ban or call it into question. This Klingon context is why April, who we know is caught up in war planning of various kinds, is so passionate that the ban exists "to prevent genocide" -- he's not thinking of people like Una, he's thinking of the near-genocide they suffered at the hands of the Klingons.
This theory still doesn't paint the Federation in a positive light, since they have effectively invented a false propaganda story to defend a policy that has led to demonstrable harm. But it makes a little more sense, at least to me. What do you think?
If you want to make the argument that people should be able to modify themselves I 100% back that up. In fact I think that probably would have fixed my problems with this episode, if this is something more like a rite of passage that Ilyrian adolescents choose to undergo then the whole thing gets way less ethically messy because now you're letting people make decisions about themselves that are socially influenced instead of having decisions made for them that are socially enforced. But I have really strong aversions to society deciding what types of bodies and minds even get to be born.
From my limited understanding genetic issues are caused by something going wrong, so a scientist can tell you exactly what went wrong and what are the side effects. For example maybe at point X you should have some gene ABC but you had bad luck and you have ABBC and this will cause issues.
So IMO if your child has such an real defect that is 100% scientifically proven that is not normal but an accident that happens very rare then as a parent you should have the right to decide to fix it or decide to let the child to live with it and maybe cure it later if possible.
I do not think we know if Bashir had such a defect in the DNA or he just inherited some "bad" traits from both parents and he did not get a cure but enhancements.