An essay nobody asked for incoming.
So, I've watched a few of those "who would win? Kuvira in her prime and Toph in her prime" and I need to get this out of my system.
Toph wins in every category and scenario unless Kuvira somehow manages to break all of Toph's limbs simultaneously.
Toph has been shown to have superhuman raw strength. This little 12 year old girl moved whole palace staircases by herself, destroyed tanks by herself, she could produce giant earth walls, avalanches and in general move giant amounts of earth with precision. She held a whole factory from collapsing while she was in a cave underneath it (comics). SHE HELD THE WEIGHT OF THE WHOLE OF WAN SHI TONG'S LIBRARY BY HERSELF. And she did that while being grounded in sand which she hadn't mastered by then so the terrain was to her disadvantage. In other words, she held a multi ton building by herself by pushing her hands into only the tower of it, countering the force of a fucking spirit vortex.
Toph has seismic sense, which clearly gives her an advantage. I do not know if Kuvira does too but even if she does, even Toph's daughters have to first remove the bottom of their shoes, stomp the ground and then focus to properly use it while Toph could literally be standing on her toes and "see". She can feel something as small as an ant from quite a distance and could sense Azula, Mai and Ty Lee on their very fast light-footed lizards from miles away. She has also been shown to feel and be able to bend earth and metal that she isn't directly in contact with.
Toph invented metal bending. Everything that is known about it comes directly from her. Kuvira may be a prodigy but Toph is the OG.
Toph took down whole fire nation air ships metal bending. While Kuvira has agility and percision, I don't think she matches Toph here.
Toph was a master earth bender by the age of fucking 12 having learned to bend from the original masters. She held her own against Bumi, a master and a member of the White Lotus who had 100 years more than her to master his skill. "Oh, but Bumi wasn't trying to hurt her". What makes you think that Bumi, who was ready to drop half an arena worth of earth on his childhood best friend who wasn't an earth bender, would hesitate to try directing half a mountain on someone who he knew was a master earth bender themselves.
Adaptability and fighting style:
The only category where Kuvira has the upper hand is speed and the fact that she can see. But speed comes at a price of burning out. Kuvira is dependent on her own body to move while Toph uses the earth to move her saving energy. She could easily counter Kuvira's attacks for a long time and tire her out to then strike a few precise shots that would take Kuvira down.
Kuvira may be fast and precise but she isn't too tactical. She doesn't wait for the right moment. She just attacks. Much like Korra. Kuvira did hold her own against Korra but Korra wasn't at her full strength then and Korra may be a powerhouse but she also isn't tactical or patient. She depends on brute force. While Toph is insanely strong she is also tactical and patient. She listens, she adapts and strikes at the right moment.
Since we're at adaptability, not only did Toph invent and master metal bending. She had one bad encounter with sand and in the span of a few months managed to perfect her sandbending to a degree where she could make a copy of Ba Sing Se from sand in exact detail. She learns and adapts exceptionally fast.
Toph's fighting style was even back in her day different from the other earth benders. Kuvira probably has not encountered such a fighting style which, again, gives Toph the upper hand.
And finally, the circumstances in which they've developed their skills:
Kuvira is an excellent bender and fighter. And I know she was an orphan and probably had to learn to defend herself early on. But take this in account. Kuvira, after being adopted by Suyin, grew up in one of the safest cities in the world during a time of peace. Most of her fighting, presumably, was training, and most of it probably earth benders and the fights that weren't training, she didn't face master benders and benders who were actively trying to murder her. Toph grew up in a time of war. Forcing her to face all types of benders, even the Avatar himself and most of the time it was trained fire nation soldiers with world domination and murder on their mind.
Kuvira fights to prove a point, Toph fought to survive. That is a big difference in regard to determination and knowledge when it comes to fighting.
Hell, even in her 80+ year old my-back-is-killing-me days, I still think Toph would win. People seem to conveniently forget that beside maybe Sokka, Toph is the proudest, most stubborn son of a bitch to ever live. If provoked enough, she'd pull a Bumi, straighten her own back, point at Kuvira and say "Heard you were talking shit" and everybody, including Kuvira, would shit their pants seeing that a monster has just been awakened.
Thanks for coming to my Ted Talk