The other thing which strikes me is that I wonder whether there is any Soviet composer who substantially changed his style around 1990 - the only issue I note is that some composers began to develop a keen interest in religious pieces, but I couldn't point out somebody who really made something different afterwards.
It's an interesting question! You would expect there 'should' be such a composer, but in reality it's hard to find one? I think there are several factors which explain this absence, though? Firstly, as you already eloquently expressed, there was not such 'tight' repression in place, and already from the Gorbachev-1980s there was little effective censorship (witness, for example, the 'ban' on music by Richard Strauss etc was lifted, Wagner operas were performed again etc). Secondly most of the composers were taught by the composers of the previous generation, with no new influx of new traditions. (It's surprising, for example, that few classical music lovers in Russia today know much about Boulez, Messiaen or Stockhausen). Thirdly, already from the late-1970s composers had been emigrating to other countries if they thought they would have more chance for expression there. Those who didn't were - mainly - in well-paid conservative jobs as directors of institutions (for example, Boris Tchaikovsky) or opera-theatres, who realised their work might be unwanted abroad anyhow.
I think the real 'change' in the 1990s was the number of composers 'from the wrong side of the tracks' who came into acceptance - who might otherwise have continued to exist in an underground hinterland they accepted as the 'price' of their artistic freedom. For example, Sergey Kuryokhin - a classically-trained pianist who'd had a rock-music career - began serious projects like 'Art Mechanics' which might have been vetoed before. Sadly he died in 1996 before the project got very far :( If the USSR was unaccepting of non-tonal music, then it was even more unaccepting of avant-garde and experimental projects like Kuryokhin's. For an example of his 'serious' music, listen to the soundtrack for the film "Mr Designer" (Gospodin Oformitel), made in 1989.
The other problem was that funding for music of all kinds dried up in the 1990s, so the great projects which 'ought' to have happened after the USSR imploded either never happened at all, or happened in underground clubs and basements like Tam-Tam in SPb - of which no record now exists :( Although the 90s ought to have been a time of experiment and freedom, in reality they marked a severe retrograde step into conservatism in all areas of music... rather typical for a period of economic crisis, when people reach out to the known and unchallenging. Thus all the soviet operas disappeared from the repertoires of Russian theatres (Rozhdestvensky famously losing his job at the Bolshoi for daring to stage Prokofiev's THE GAMBLER), to be replaced by a diet of ONEGIN, TRAVIATA and CARMEN.
Post-1990 composers have also struggled to write in new genres which didn't exist in the USSR - notably musicals. Serious composers have put their pens behind new works, but I can't say that the results have been very good - mostly they resemble pseudo-Sondheim :( I wish they would stop aping Broadway models, and write with the conviction of their own style?
It isn't entirely gloomy, though - there have been post-soviet masterpieces in a clearly post-soviet style... the best of which has probably been Desyatnikov's ROSENTHAL'S CHILDREN at the Bolshoi - a work which they mysteriously refuse to tour abroad?
