Different situations may require different solutions. Splitting the heading dropdown into separate buttons gives the user quicker access, but at the same time takes up more space. It is up to you to decide whether easier accessibility is more important than a tidy toolbar.
Usability and editing needs are not the only factors. Sometimes the user’s personal preference and previous experience are equally important.
Whichever you choose, CKEditor 5 offers both the standard heading dropdown list and separate heading toolbar buttons. This way you can meet your users’ individual needs.