About Tosen...

I'd guess it had something to do with the Hollow powers. Aizen might not be able to obliterate to Urahara and Ichigo because he didn't hollowfy them himself

you could say the same for as to why he never showed Ichigo his Kyoukasoigetsu. The answer? because if the villain did everything and I mean literally everything to win with out making any mistakes... someone like Aizen would win 100% of the time and we wouldn't have a story.

However, putting story logic aside.. I suppose it would've been too easy for Aizen to do so. He's not the kind of person that would do something so easy. He likes to toy with his enemies, he doesn't need to use his full power to defeat the entire soul society.. at least not until he's achieved his goal of breaking the barrier between soul reapers and hollows. Then, and only then, will he finally kill them. That's the kind of guy he is.

You say he sucks at balancing op characters but it's never been confirmed that Aizen even has a bankai. It's theorised that he never achieved it. During captain selection, it would've been easy to fake one using hypnosis.

But like I said before, it's not in Aizens character to go full out. You say it's bad writing but he takes enjoyment slowly beating his opponents. Bad writing would be to just do what you suggested and to just obliterate his opponents. Kubo creates depth with his characters by them not going full out straight away. It's just one of many techniques that he uses. Especially with Aizen, he let's him grind his opponents down physically and mentally before he finally would decide to kill them all once he's achieved breaking the barrier between hollows and soul reapers.

Obliterating his opponents instantly would be horrific story telling. In fact, we wouldn't have a story because it would be over that quickly so idk why you're suggesting it.