Short answer: yes, this is fine. It would be absurd and pointless to make you wait until after the conference to submit the journal version of the paper. Even waiting until after you get the decision about the initial paper’s acceptance to the conference has no logical purpose that I can think of, and is unnecessary IMO.
(Of course, the journal version of the paper should cite the conference paper, and maybe discuss in what ways it’s expanding on it.)
Detailed answer: in computer science, some areas of math, and possibly other disciplines, it’s very common for authors to submit a preliminary, typically short version of a work to a conference and later prepare a longer version to submit to a journal. This is seen as completely normal and acceptable. The rationale behind this is that the initial short version is a quick way to disseminate exciting new research, while perhaps not communicating the work in as much detail (or in some cases as accurately) as the slower-to-prepare version; whereas the long version is supposed to conform to higher standards of detail and accuracy, and is peer reviewed according to the more thorough and strict standards of academic journals - so that represents the “ultimate”, more authoritative, version of the work.
What you are proposing to do is completely in keeping with this philosophy and the norms around this sort of dual publication pipeline. In fact, I would say that you are maybe being overly cautious in suggesting to wait until your conference paper is accepted before submitting the journal version. I don’t see any reason why you would need to wait at all - you can submit the longer version as soon as it is ready.
The only type of related behavior that I would find problematic is submitting the journal version before the conference version. As the journal version is supposed to supersede and obviate the need for the conference version, personally I see this as a type of gaming the system (and akin to a kind of double-dipping or self-plagiarism), at least given the norms prevalent in the specific fields I am working on. Actually I was involved for a few years with the program committee of a large math conference, and encountered a situation where authors submitted a conference version of a paper that has already been submitted and accepted to a journal. I was not involved with the final decision-making on this paper, and I vaguely recall that the people in charge did decide to accept it to the conference (and they had some reasoning why they thought that made sense, which I thought had some validity but was not very compelling). I expressed concern at the time that the paper shouldn’t be accepted, and I know I wasn’t the only one holding that opinion.