I wasn't familiar with dialogue acts, and from What is the difference between Speech Act and Dialog Act?, dialogue acts appear to be a construct favoured in computational linguistics.
Speech Acts are about what you do with language, as a social action, through an utterance: making promises, giving instructions, expressing emotion. They are in the domain of pragmatics. Topics are about what subject matter an utterance relates to: they are in the domain of semantics (potentially narrowly truth-conditional semantics). If you go back to Austin's original formulation—
- A locutionary act, the performance of an utterance: the actual
utterance and its ostensible meaning, comprising phonetic, phatic and
rhetic acts corresponding to the verbal, syntactic and semantic
aspects of any meaningful utterance;
- an illocutionary act: the
pragmatic 'illocutionary force' of the utterance, thus its intended
significance as a socially valid verbal action (see below);
- and in
certain cases a further perlocutionary act: its actual effect, such as
persuading, convincing, scaring, enlightening, inspiring, or otherwise
getting someone to do or realize something, whether intended or not
— the topic belongs to what Austin calls the rhetic act (producing a semantically coherent utterance), which is a subset of the locutionary act (saying something). It is on a different plane of analysis from the illocutionary act (what you intended to achieve by saying something).
Conveying meaning about topics is only one of the many possible speech acts, and having relevance to a topic is not a necessary condition for there to be a speech act. On the other hand, if your speech act is assertive, it is intended to convey meaning about some topic; so the existence of that topic is presupposed.