https://www.webnovel.com/book/10343178605010205/I-Won't-Kill-Myself-for-the-Second-Time
Give this one a try. It starts with a cliche of waking up in someone's body but there's a lot of mystery at the beginning and it deviates from it around ~5-10 chapter and by 20 chapter fully spreads its wings. What's more, it should be exactly what you want:
Mc personality is bad, but he tries to change slowly.
Mc starts not even close to the word op, more like zero.
The action is set in a slightly alternative modern world.
But remember
There's no cultivation.
Forget about marial arts.
It's a mix of slice of life, action, mystery, fantasy, supernatural
I don't want to spoil too much, but check out some of the reviews if you're not sure if it's worth giving a try.
I wrote this novel exactly due to the reasons you've mentioned because I was bored with super op mc's from the very beginning, flat characters with no development, mc's 'yes man' army following him, and winning every challenge on the way without breaking a sweat.