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.

This depends on what your institution values in its faculty. It also depends on what you would need to give up that might be more important to obtaining tenure. If it comes at the expense of "a few" papers at a place, such as an R1, that highly values papers, the risky. If it comes at the expense of grant proposals at such a place, then risky.

"Not egregious" can mean a lot of things. Tenure should be your first concern at this point if you intend to stay in academia.

But if the institution values such things as having its faculty present a public face to the academic world, maybe a liberal arts college, then less risky.

But, on balance, I think your guess that it is better left for post-tenure is the right one.