Have we ever seen Koshimae holding back? Sorry, but just like Sanada, Koshimae doesn't know how to hold back either.
(It wasn't an official match, so I don't think we can assume Ryoma did everything he could, they were probably just trying to out-rally each other.)
Really? Koshimae played a one point unofficial match with Kintarou, which he didn't intend to play. Was he holding back? So, why he will hold back when he cant defeat Kazuya?
I'm sorry but Ryoma holds back all the time, and he is NOT like Sanada lol.
Against Matsudaira Chikao, he was playing around.
He played with his right-hand for a long time against Kirihara Akaya, Ryoma switched from right-hand to left hand against Tanishi Kei,
He went up against Atobe refusing to use Muga from the start,
There are many examples I can bring up where Ryoma is a cheeky brat who holds back for fun.
Your example for his match against Tooyama can't really count because Konomi has Fuji mention how they must feel that they are destined rivals where both refuse to lose to the other. Tokugawa and Ryoma aren't "destined rivals" like Tooyama and Echizen at all.
I don't think there is any evidence or implication that Ryoma went and brought out Saiki, Hyakku Ren, PoP or even Standard Muga against Tokugawa.
They were just out-rallying each other and of course, Ryoma and Kintaro were completely out-rallied.