Sure. Pardon if you are aware of this, but I will lay out what I have to advise most companies below.
I suspect that you historically were sending all email from one domain: yourdomain.de or similar. This setup can cause many issues with deliverability over time, which it sounds like you are now seeing. Fixing it is not quick but can be achieved easily.
Business and transactional emails need to be separated from marketing communications by sending domain
Business (Customer service responses) and transactional (Order confirmations) send from yourdomain.de, but marketing communications send from another domain or subdomain: email.yourdomain.de as example. (Check your spam folder at your personal inbox and you will see that most companies follow this practice as it protects you main domain from being flagged as spam by email companies.)
Typically, you would only configure Brevo and their servers to handle marketing communications since they are usually larger volume and more likely to be flagged as spam.
Your reply-to address in brevo can be a transactional address
You can keep your transactional communications coming directly out of TB if you'd like using smtp. Brevo can handle these as well though if you'd like but more sending = additional cost.
Ensure that you are not sending attachments in any emails by default, ever. This is almost a guaranteed spam flag against your domain. TB adds invoice attachments by default to order confirmations if you enable invoices. TB can be updated so that it still creates invoices, but does not add the attachment to the emails.
The switch to using a new sending domain for marketing will take some warming to build a reputation. Start your first few sends to known recipients so that they do not flag you as spam manually on their side to help warm your domain.
Additionally, in a different thread in this forum is a post with my latest version of the Brevo module that works with TB 1.6 and php 8+ and offers the ecommerce capability sync. If not using it, i advise testing on your site... it works really well to keep order data and lists synced to brevo.