I find Russia/China dynamics pretty interesting, but from my experience Rochu is one of those pairings susceptible to a horribly reductionist kind of seme/uke characterisation. Just because Yao is shorter, smaller-sized and more slender in build than Ivan doesn’t mean he is totally submissive/passive or the weaker party in their relationship. He is around three thousand years older than Ivan! He has seen a lot of things- he wouldn’t be utterly scared of him.
And like historically, I’d say China has generally been wary, rather than scared of Russia. Wary the way the US is wary of China- because China is large and powerful enough to challenge Russia. So for me I kind of don’t really like the Russia-China dynamic where China is scared of jealous/possessive/ intimidating Russia. This is a dude who has seen the rise and fall of Rome, Greece, the Byzantine, the Ottomans, the Mughals- and weathered dozens of invasions. I really like that sort of ancient wisdom and unflappability to show through- and at times, unnerve the younger nations. So, in my headcanon, he’s a very tough, patient and shrewd person- and it’d take a little more to shake him up. And he is a very, very experienced lover because of all the practice he has had ;)