That's already run into problems. "Evolved enough" socially is very much set by a human/Earth standard, and even then the Federation may be accused of double standards.
Worf ran into trouble for giving assisted suicide to Kurn because he was dishonored by Gowron. However, it's valid under Klingon law. I can't imagine the Klingons would willingly give this up if for some reason Federation membership became a realistic prospect.
Or, even more shocking--the Klingons will agree to give up their rigid "honor" suicides, just as long as Federation standards are treated fairly on
all member species. And the Klingons will point out that Vulcan forcible indoctrination of emotionless lifestyles on children, and their fatal Pon Farr fights, are just as barbaric if not more so than Klingon ways, yet the Federation lets those slide.
The Klingon Empire were allies with the Federation not members (except for that short period in DS9 when the Khitomer Accords were dissolved because the Federation condemned Klingon invasion of Cardassia and Starfleet decided to rescue the Cardassian civilian council (this period was also when the incident between Kurn and Worf occurred) - which also led to the conflict between Federation and Klingon empire over the Arcanis sector.
Worf was a security officer of Starfleet. And while technically, he was probably considered a Federation citizen, there are differences on what is allowed to do when part of Starfleet and what isn't.
Not all Klingons practice those rituals after all, and even Worf himself didn't like the idea of assisted suicide to Kurn.
The Federation does respect people's religious customs, and as Janeway said, there is a limit to how far she would allow religious practices to go on her ship... because B'Lanna at the time wanted to repeat the life/death situation to revisit the Barge of Death and actively put herself (her life) in danger as a result.
I fairly doubt a Klingon captain would be so reckless to allow their crew to indulge on a very dangerous medical procedure... if EVERYONE did that, I doubt there would be many KDF officers left.
Besides, that assisted suicide ritual was also a product of very unusual circumstances.
I'm assuming that large powers like the Klingon Empire might not become part of the Federation but retain their allied status instead.
Either that, of the Empire would have changed over time when they decided to abandon a lot of those practices (remember that a lot of what we saw in the 23rd and 24th centuries was a product of rise of Warrior caste in the Klingon Empire... but non-warrior Klingons obviously exist... and we hadn't seen many Klingon scientists embracing too many suicide rituals... even the Klingon doctor in Enterprise [NX-01] was desperately trying to find a cure... albeit his methods were not exactly in line with Denobulan medical ethics - and many Klingons benefited from Federation medical practices.
The Klingons that Voyager found in Season 7 (who had the Neret virus) were similarly tempted to commit suicide upon being debilitated with Neret... however, when they were cured, that desire for suicide went away.
Klingons aren't always the most patient, but they might benefit from tempering their impulses a bit.
Looking at the Hirogen... they remained hunters even after getting holodeck technology from Janeway... the hunt continued in simulated form allowing the Hirogen to not exactly roam the galaxy like nomads.
Societies change over time... but just because that change occurred, it doesn't mean alien societies absorbed into the Federation would suddenly turn human.
No.
Certain things would change yes, but cultural identity would persevere regardless.
At any rate, how do we know that Klingons are supposed to be bloodthirsty? They were invaded by the Huur'q during relatively early stage of their development... and when that failed, Klingons stole alien technology and became space faring.
If that didn't happen, its possible Klingon social evolution would have occurred differently compared to what we saw.