The only tool I find hard to use, as a left handed, is scissors, the rest is just fine. As for the mouse, always used it with the right hand.
I also use the mouse with my right hand and I'm always surprised by how many left-handers actually ended up using the mouse with the right. It's quite strange. Even stranger, was a right-handed colleague who decided to use the mouse with her left.
I use the mouse with my right because the rest of my family is right-handed so it was on the right and I just learnt by copying. I don't mind it, as I can type fairly reasonably one-handed with my left hand - much better than I could with my right - so it means I can still do short typing bursts one-handed without lifting my right hand off the mouse. When I type with two hands, I find my left covers about 60-70% of the keyboard vs. my right doing about 30-40% of the keyboard.
On thumb keyboards, it’s pretty similar too. My left thumb types as far to the right as U/G/B.