Hi,
Some users have found you need to give it 20 seconds or so to boot up fully before trying to drive off. I just jump in and go in my Trophy so not sure, maybe speak to your dealer for a software update?
Re: LKA, it only activates when you hit 38mph - so if the majority part of your journeys are under that speed, being a taxi, you shouldn't need to switch it off every time. And even if you're driving above 38, it shouldn't do anything drastic apart from gently nudge you back into your lane if you cross the lines, beep and vibrate the steering wheel. Indicate when changing lanes and it shouldn't happen anyway.
If it does anything other than that, then I'd suggest speak to your dealer and ask for the most recent updates, as many users report LKA being better now than it was last year for example, after updates.
If it's your first car with LKA, it will take time to adjust to the car moving the steering wheel anyway, so bear that it mind too.
Good luck with it and hopefully you get some solution ?