I have to agree with Christian here, and while he is always very diplomatic, I will take the liberty of putting it a little more bluntly. 
Wanting to delete posts or even entire threads just because you haven’t achieved your goal yet is very selfish. People have put a lot of effort into giving you detailed answers, and now you just want to delete their posts.
You can delete your own posts, but I think that’s very selfish too, because it diminishes the value of the answers for others, as the context is missing.
A forum is not a private support or consulting channel. The whole point of a forum is that it is publicly searchable and everyone can benefit from the information. If you want private support or consulting, I suggest you contact Nextcloud GmbH or an IT service provider and seek their advice. However, this usually costs money.