I usually end my post with a banner like this and an opt-in box like that.
My 2 questions:
1 – How can I add them automatically to my post without having to take any risk in breaking my current blog theme and the way the posts look?
I shared 2 URLSS of my blogs in the PI field info.
2 – What about the posts where I already added them manually? I have like 25 of them or so.
1. You can add the banner and the opt-in box into a block element, select hook as the element type, select after_entry_content as the hook name, and set posts > all posts as the location.
2. Once the element is activated, the content will be added to the posts that already have them too, so you need to manually remove the content from the 25 posts, or you can add these posts to the exclude list of the element.