Would taking an Editor-in-chief position at a mid-tier journal be good strategy for a tenure track professor?
Being an editor-in-chief is a rewarding position that most departments value on someone's CV.
So much for that. But I cannot stress enough how bad an idea it would be to take on such a role while being an assistant professor without tenure. As the EiC, your workload consists of 50% assigning papers to associate editors, and 50% dealing with the difficult papers where some bigshot's poorly written paper has been rejected and they complain/some associate editor not actually doing their work/other cases that are not of technical nature but involve people. You have to make judgment calls about these papers, decisions, people, and that's hard if you don't have the stature and independence that tenure gives you. That's because to get tenure (and, to a lesser degree, for promotion to full professor) you rely on the well-known people in your community that are exactly the ones who you might have to make decisions about. In other words, without the independence that tenure gives you, you will perpetually find yourself caught in conflicts of interest, and that's a really awkward position in which you can only choose between lose-lose options.
In other words, don't do it if you don't already have tenure or anticipate getting tenure shortly after taking on the role.