Is the following publishing strategy convenient?
As you describe it, it seems like standard practice (more or less). Conferences prefer somewhat shorter works than journals due to time (reviewing) and space (proceedings) considerations. Journals don't have that limitation, though some won't publish long works. 

You will need to be careful about citations of your own work, of course and you may have issues with the possibility of double-blind review in some situations. 

However, if you publish the journal article first, there would probably be a presumption by the conference committee that it is "old work" and not suitable. But, in general, extending an older work is acceptable - even praiseworthy. 

But, to satisfy journal reviewers, the "extended" work should be more than a restating of the earlier work. A summary of older work might not be enough, even if it brings together the, already published, algorithm, and the, already published, use. Reviewers may be looking for what is new.