Flanking doesn't just not work at the moment, it doesn't work in general because of the core design of the game.
And thats a totally sad thing because "True Sight" is perfectly made for flanking moves. I completely agree that it is pretty hard to get a successfull flank because you have to get to that small 90 degree area behind that tank while there are a lot of other units around. This gets especially hard because both M10 commanders have no form of rocket artillery or good offmap that helps to strip that support. Abilities like Fear Propaganda Artillery or White Phosporous Barrage would be a good addition to a commander with flanking tanks to supress/slow/conceal that support. Alternatively any form of rocket artillery could work. A partly snare immunity and more received acc for M10 would help a lot. You can always bind it to an ability to balance it.
I do think at directly fighting the StuG G the M10 sucks, compared to M36 it sucks too. Lets not speak about Panther. You can't make it viable in frontal combat without a lot of buffs. But there is still some room to improve M10 for flanking as long as it looses the frontal fights.