"Also I've done some research, and found many landing page and website builder tools, some of them are even free." Yes. THe question is: do you want to compete

"Also I've done some research, and found many landing page and website builder tools, some of them are even free." Yes. THe question is: do you want to compete with Wix, Squarespace and WordPress ? When you say "custom web and mobile app", how do you know that Smaller businesses really need that consistently. Most small businesses are happy with a website with Wix/Squarespace and don't really need custom web apps. So you need to go and do some customer discovery. The question is not whether you can start a web design company, the question is who will you target and what can you offer that is not already been offered as a commodity.