I got around 900 matches with MiG-23ML to grind the Soviet Tech Tree, it’s massive but when I learned about the plane and started to get more efficient with it, the boring farm was way less stressful.
But if you don’t enjoy the plane you are grinding, set a daily amount of RP to research and then switch to another planes of the same tech tree, perhaps play some AB too for relief.
Yep. I recently spaded Kongou and Haruna together, and between the two of them I got ~115,000 RP towards Mutsu for just… playing the game and spading two vehicles.
As a not especially useful but interesting fun fact, spading bonuses are not affected by Rank efficiency; that ~115,000 would also have been ~115,000 towards Rank I ships, had I been researching them.