First suppose, Russia keeps supporting Iran against Saudi Arabia and Israel. Saudi Arabia could not only get nukes for itself but also with Pakistan and Israel get them for Turkey, Ukraine, Estonia, Finland and Sweden. The Saudis working with Pakistan and Israel could ring Russia’s borders with nuclear weapons in hostile states to Russia.
Ukraine with nuclear weapons has nuclear reactors and know-how and could gear up especially with help from Israel and Pakistan. So Russia could be ringed on its borders with nuclear weapons.
A basic principle of nuclear strategy is to avoid countries on your border getting nuclear weapons. That includes Iran.
Pakistan is allied with Saudi Arabia, so if Russia goes against Saudi Arabia, it is going against Pakistan. Iran is an ally of convenience. So Russia is building a nuclear wall around itself. This is repeating China’s mistake.
If Russia did a deal with Saudi Arabia and Israel to betray Iran, it removes a potential hostile nuclear state on its border. It also prevents the spread of nuclear weapons to Turkey, Ukraine, Baltic Republics, Finland and Sweden.
Saudi Arabia could also reduce the supply of oil on world markets and raise the price of oil to help Russia. So Russia has a lot to gain by doing a deal with Saudi Arabia and Israel to stop Iran getting nukes.
This is draft and preliminary. The above is hypotheses and speculation. Comments and corrections welcome. Please restate as questions. All other disclaimers apply.