Your advices on CI3 upgrade

That's a hard decision to make and I don't have a magic solution. But from my understanding, the reason CI4 is so much different from CI3 is because it was considered obsolete by today's standards. So it was rewritten from scratch. I'm sure the next major version won't have that much breaking changes. Yes you have to rewrite your application, but I don't think it would take as much time as you think. If  you are not familiar yet with CI4, I suggest you make some test applications to learn what you can do and how to do it. Then create a new application with CI4 and start building your marketplace. You can probably reuse a lot of things. All your HTML, CSS and JavaScript can be reused almost as is. You will have minor changes to do, but nothing too big. I think the bigger changes are with the models, entities, controllers and filters. You will need to rewrite your queries to use the new syntax, but the SQL part is the same. Maybe you can hire some people to help you to do it faster. You can take a look at my tutorials if you want to have an idea of the basics.