Why would Aizen fight Yama straight on Aizen said it himself more often than not it was likely Yama would win also till his first evolution Aizen wasn't getting any real boost from hogokyu after his first evolution he was above Yama by a lot so was Ichigo by the time he fought Aizen later.
And again I gave all the context Yama sure is a goat but 80 percent Yhwach with his powers still not complete and without all mighty is around Yama level remember bro Yhwach who had already got his almighty and most of strength sealed is what lost 1000 years ago so yeah while Yama without Arm was weaker Yhwach was way way weaker than his true strength and Yhwach didn't need his clone bro if you read or re-watch it was about making Yama feel hopeless Yhwach would win in a fight but nah that wasn't what he wanted he wanted to humiliate Yama and also tell him how his ideology was worse.Yhwach is a petty dude.
Yama can be the goat and we can still have nuance and actually scaling it sucks because Yama should have been allowed to burn of Quincy thugs Yama didn't get to do his ****.