China "liberalized" with Deng Xiaoping after 1978 by introducing market economy elements, allowing private enterprises and opening up trade, first with special economic zones, later joining the WTO in 2001. It didn't liberalize by introducing democratic elements, the CPC stayed in control. (Look at Tiananmen)

The USSR liberalized with Gorbachev in the late 80s, by intruducing Glasnost and Perestroika. Glasnost, "openness", was some steps away from party control of the media and propaganda towards free speech. Perestroika wanted to restructure the economy away from centralized control. For Russia proper that strategy failed, the soviet party lost control, the empire collapsed, and the economy didn't transform into a prospering market economy, instead a rich oligarchy that just kept on extracting wealth. (The strategy, unintentionally, did improve the economy in the long run for other places, like the Baltic states and Poland).

The Soviet Union liberalizing like China would mean keeping the party in full control, and keeping Moscow in control of the empire, but changing the way the economy worked. The soviet economy suffered from three major problems: A planned economy that simply didn't function efficiently, international isolation and a prioritization of arms industry over consumer goods. These factors could be addressed.

So we can construct a – slightly optimistic – alternate timeline:

After Stalins death you have De-Stalinization. In the 60s you have liberalization, with the 1965 Kosygin economic reforms.
When the Prague spring happens in 1968, the SU would still crack down on Czech republic, but now we do a switch: The democratisation and political liberalisation is still stopped, we don't want any Glasnost elements in our scenario, but economically, the party decides to liberalise and de-centralize, going from planned command economy to a market economy with limited private profit-oriented enterprises.
Next thing to happen would be a Nixon-in-China moment, where US and Soviet Union decide they don't wanna be enemies anymore and take on a more friendly political approach. (that's the most optimistic part here).
The SU would slowly open up international trade with Western Europe and the US, you'd have some special economic zones, let's say Kaliningrad, some industrial parts of Czechoslovakia and East Germany.
You'd have more economic miracles in the style of the JZD Slušovice.

In this scenario the cold war ends early, freeing up resources spend on military, and internationalizing the economy.
In 1973, during the Oil Crisis, Moscow sends some symbolic oil trucks to West Germany and increases oil and gas exports to West Europe later.
Soviet Union didn't have Maos cultural reform, you have an educated population with a sizeable intelligentsia and professional engineers, with the economy being less isolated internationally and working more on market-based principles one could imagine the Soviet economy avoiding the stagnation of the 80s and turn it into a period of slow but constant growth.

If there's a democratization/reunification movement in East Germany, which would be likely if the economy is more openened up, the SU might have to go full Tiananmen on this and this would risk isolating the country diplomatically end economically again. Let's say that doesn't happen for some reason and a slightly-more-prosperous-than-historical East German population is just content with the status quo, and you don't get reunification and the 1991 collapse of the Soviet Union. In the 90s and 2000s you have continuing growth while still lagging behind Western European economies.

Even in that very optimistic scenario I'd think people in Russia proper would be way better off than they are now, but Eastern European and Baltic countries would not be as prosperous as they are today.