1989 is too late. I mean, you can save the USSR just by counteracting the coup and gulaging Yeltsin for working with the US easily enough, but to really fix the problems with the USSR, you need to go back to the 1960s, 1950s.
Personally my views is that there should have been much more focus on creating a Socialist "pop culture" and cultivating artistic freedoms in the USSR. The USSR should have been exporting it's culture, instead, everyone just imported western culture then the USSR did it's bad knockoff 5 years later.
Revamp the party structure. Nomenklatura needed to be done away with and Party's appointment authority and Patron-Client Relations inside the party structure completely scrapped. These systems were literally just institutional nepotism, which saw talent lying at the bottom of the scrapheap while those who sucked dick of the right "patrons" made it to the top of the party. There needed to be more democratic measures implemented within the party itself.
Market Socialist reforms in the 1960s. No large Capitalist enterprises, but I would allow small businesses of say 10 people and then beyond that worker cooperatives. Have a regional development bank in every Oblast to fund decent small business and Cooperatives.
Better trade deals with other countries such as Japan who were manufacturing power houses and right on their fucking border.
Far more investment into consumer goods and IT. USSR computer technology was a decade behind where it should have been. The USSR had some of the best scientists, including computer engineers in the world, the USSR should have been a tech-power house, instead they relied too heavily on post WW2 Heavy industry.
Lysenkoism needed to be purged much more quickly, there is no reason why this nutters ideas hung around until the 1960s.
Also Soviet politicans just needed to listen more to experts and scientists. They actively seemed hellbent on sabotaging the country sometimes. Look at Khrushchev's corn disaster, completely ignored agricultural scientists and experts telling him Corn required warm climates, instead he tried to farm it all in Siberia, The Baltics etc. The fuck?
Change this and the USSR may survive. Too bad 99% of the party will oppose you.