GDPR (DSGVO) relevant concerns for website

we would ask for your professional assessment of data protection issues that arose during the development of a website.

This is intended to establish, mediate and manage networks through which locally produced goods can be displayed and distributed.

For this purpose, a user can register on our website with an email address and username, set up a network and provide key data about his offer, which is then displayed to all other visitors. A contract between the provider and a registered consumer is not explicitly managed by us. However, we do provide a chat that users can use to clarify everything else.

There is also a public blog for each network, in which only certain people are allowed to create amounts, as well as a newsletter from us that everyone can subscribe to.

All of our services are free of charge, but a PayPal button for voluntary donations should be implemented.

In the course of development, the following questions came to mind:

1. Do you have to pay attention to something special when implementing a chat, for example special encryption?
2. Should a user be allowed to delete his posts directly and permanently without us being able to restore them within a certain period of time? We ask this question because it makes it impossible to follow up on abuse and we want to avoid a conflict with other laws, if such exist. Especially german ones.
3. Is it enough for users to agree to the data protection declaration when they register and then post without further consent, or do we have to obtain consent in any case?
4. How do you handle the case that a registered user revokes his consent to the data protection declaration? Does the account then have to be deleted, or can it be treated as "inactive" and the user given the opportunity to consent again when they log in again?
5. Can you send the users of a network an email notification by default about a new post in the network, giving them the transparent option of deactivating these notifications?
6. Under what conditions can usernames be shown publicly?

Thanks in advance for the help!