Not just the nuclear-early-warning system, I think they should cooperate with space based assets as well.
Once the ISS is de-orbited (stations don't last forever) Russia and China will cooperate in a space station. Russia already announced their intention to do so. Russia cannot afford a large space station by itself but they have a lot of elements they can contribute to a space station. For example they have some of the best water and air recycling systems (technology also applicable to submarines to an extent). There was also the joke about how the US toilet on the ISS broke and everyone had to use the Russian toilet. Well, there are lots of little things like these which won't be apparent until China tries to have long endurance missions (months or years) in space.
I think they should also cooperate in global satellite networks. I know Russia has issues with often only partial networks in place. China also has a much better launch site (closer to the equator) than Russia. For example there is no good reason for having both GLONASS and Beidu unless they can't cooperate for some reason. If both nations have access to a military grade signal this could be a place for cooperation. In exchange Russia could grant some of the technologies they have for reconnaissance satellites for early warning for example. Or they could have an agreement to share satellite intelligence together.
You have to consider that there is more to being a reliable partner than just giving all the technology away. As long as Russia is a reliable trade partner then they can produce some items and sell those to China. That provides them with resources which they can invest into R&D. Had it not been for sales to China and India, Russia would have had issues funding further T-90 and Su-30 upgrades for example. Same deal in reverse. Russia, once China has its own nationalized chip production equipment, may either import Chinese equipment or even fabricate government chips in China (SMIC) instead of Taiwan (TSMC). With a reliable manufacturing partner for chips, Russia could invest more on chip design than they do today, with a local plant with similar technology and processes in a smaller scale, they can do component testing for mass manufacture in China.
Russia has other technologies which it exported to China we did not discuss here before. For example Russia transferred the technology for their 6th generation uranium enrichment centrifuges to China. China already improved on that technology and made their own versions. Russia also has a later generation version for it. Russia has exported their fast sodium cooled nuclear reactor technology to China. Little known fact, Russia is one of the main world producers of carbon nanotubes. They are now starting their own production lines for large composite structures (Japan is the leader there) and one example is the wing of the MC-21 or the skin of the Su-57.
Russia has contracted Chinese companies to build the Moscow metro extension and are also considering doing the same for high speed rail. It will be difficult to build a network as extensive in Russia though. Other than the Moscow-St. Petersburg axis the Russian population outside of the European side is quite sparse. In that case aviation is more suitable and Russia is building an extensive and renovated airport network for that. I already mentioned possible Russian-Chinese cooperation on the CRAIC 929 and if that is successful I think similar collaboration could happen for either a single MC-21/C919 successor aircraft design with twin production lines or the CR939.
A CR939, think Airbus A350/Boeing 777 competitor, would provide both nations with experience in larger aircraft and with the technology to make larger engines which would be useful not just for the airline business, but also for stationary power (think gas turbine power plant), or marine gas turbines for large naval vessels.
Another technology China could benefit from with collaboration with Russia would be certain high energy electronics. For example the Russians manufacture the Linear Transformer Drivers (LTD) used in the latest iteration of the USA Sandia Laboratories Z-Machine. This is a so called z-pinch fusion facility. AFAIK the Chinese don't have a facility like that and the LTD is basically the main component in building it. It is a device used to trickle charge a high amount of electricity and then quickly discharge it in a fraction of a second. It replaces the Marx generators previously used in the Z-Machine. If China imported that technology and built a similar facility it could not only help them with fusion power experiments, it would also help them model fusion explosions, for nuclear weapons stockpile maintenance and warhead design.
Another area of collaboration: Russia is modernizing its naval shipyards and it is using Chinese equipment to do this. One example is the Zvezda shipyard in the Far East. Russia has a long shoreline to defend so having a decent enough navy is paramount.
You mentioned cooperation in tank design, well, I am less sure about that. I think the design requirements are kind of different. At best there could be component sharing but I think each country has its own design school. With regards to helicopters I think each country has opted for a different infrastructure in terms of powerplants which means technology sharing will be difficult.
