One thing I realized later that I did wrong was pushing back on the upper management too much. For example I said I did not want my team working weekends on regular basis as had been the practice up to that point. We ended up not having to anyways but it painted the team in a bad light.
Really this translated into me taking a long time to understand what the priorities of the business were vs the development priorities. Definitely let's of stuff I would consider typical first time manager mistakes.
If by working the weekends you mean seven-day work weeks on a regular basis, I strongly believe you were doing the right thing.
The health of your employees is a much bigger priority than business or development. They're not human resources, they're people with families and loved ones and the need to have a day off. Unless you were on the verge of curing cancer, there is no reason to work people like that.
You were in a bad situation - it sounds like there were huge problems with upper management - and you did the right thing. Unfortunately doing the right thing doesn't guarantee success. No doubt you made a lot of mistakes too - I've been there and it hurts - but that doesn't sound like one of them.
To me it's a sign in your favor that the team left after you did - if you were the problem, they would have left before you did. I don't think you can expect them to be happy when they're working weekends and (what sounds like professional developers) are doing manual data entry for weeks on end.
More like working Saturdays for weeks at a time. My personal rule of thumb (after volunteering to work until 2am on Saturdays and then again Sunday mornings to finish a project and launch on Monday, just to have it delayed by another three months) has been to work a weekend once a year. I think that's about the frequency with which true emergencies happen, the kind that affect the bottom line and the livelihood of the organization. Anything more than that and it's just someone trying to squeeze more out of you than you can give.
In this case I took a stand and while I think I did the right thing by saying that I did not want my team working with this pattern, I also misrepresented how dedicated my team was to finishing the product. This created quite a bit of conflict between upper management and the dev team.
There is absolutely nothing wrong with sticking up for your team. Management does not get to foist their lack of vision/preparation/thoughtfulness onto subordinates in the form of, "but we need this yesterday."
How many business functions are actually so important that they require weekend developers?
Sounds like the company was already pretty dysfunctional if your team was working weekends on a regular basis. That's not healthy, and if you were my manager, I'd thank you for standing up for our lives outside of the office.
Really this translated into me taking a long time to understand what the priorities of the business were vs the development priorities. Definitely let's of stuff I would consider typical first time manager mistakes.