I'm interested in the idea to use 5 and not version 6.

But I should advice to start with 6. The difference isn't very big but you have the latest options.

We agree then, but you maybe able to describe in details the difference better than me, supposing for example that I would use plugins or solutions developed by the community, how much you think I will face a problem in finding options that works well with 6, or any plugins that were built for 5.8 can work for sure with 6. the is exactly the question.

Laravel 6 as I know has come with enhancments for the code organization and better practices, do you think this would affect the integrability of 5.8 compatible plugins with 6?

You can have a look at what has changed in this version here so far I have had no issues with incompatible packages and if you do find them you should be able to fix the issue by looking at what has changed and sending a pull request

Im working on a big project at the moment on laravel 6 and building packages

To be honest if you use packages that aren't available for Laravel 6 they are probably not maintained anymore. And on that moment you need to check if you want to maintain it (and then directly add support for Laravel 6) I shouldn't add unmaintained packages to a project.