Did the USSR really believe they could take over Germany and Western Europe in a conventional war without risking NATO nuclear retaliation in the 1970s and 1980s?
No, and they didn't think they could/would/should do it.
THEY were worried about the West invading THEM at the same time WE were worried about them invading US.
This. Maintaining the Soviet fear of a NATO nuclear response in case of invasion is also why the West always fought efforts to get them to renounce first use of nuclear weapons, an otherwise sensible idea. Renouncing first use would erode the credibility of the nuclear deterrent that was an essential part of the NATO strategy to prevent an invasion of West Germany.
Post collapse the East German Army handed over all their documentation to the West German/NATO military. That included five Warsaw Pact current contingency war plans possesed by the East German Army. All five plans assumed the use of nuclear and chemical weapons from the first hours of any major conflict.
A West German army Lt Col who took command of a East German artillery regiment for the assimilation period noted that the center piece of the regiments training was rapid evacuation of its barracks to dispersed war positions. They were able on zero notice to get all their combat equipment out of the maintiance parks and the specified distance away in less than a hour. He also discovered the self propelled howitzers and ammunition carriers were always loaded with war ammo, even when in the maintanance areas.