What would be a good strategy to avoid being scooped?
Submitting the manuscript to institutional repositories, and if the field permits it, publishing at arXiv, before sending to a journal are the most common ways to guard against scooping. Another line of defence is storing all communication with the journal you submitted to, so that if any malpractice is found later, you can bring it to the editor's notice and get justice.

The strategy you suggest is flawed based on two reasons:

(1) Many high-ranking journals will not touch a fluffed up version of something published in a low-ranking journal. It is possible that it may not even go to review. There is a high premium on brand-new, original research now; I have come across cases where high-ranking journals reject papers that even use data from previously published work.

(2) This doesn't really prevent someone from scooping your work. As you said,the first publication will not have all the details. Someone could still use your work and claim that they independently expanded/improved upon that initial version.