Understanding Taxes and Cost of Living

Hello! I'm getting lost in my own research and AI results, so I thought I'd post here to hear from actual people.

For context: I'm a family of five with three young kids. We are wanting to live somewhere near-ish the city center. We are support raising in the US right now for our ministry, and will be living off of that (and possibly part-time local work). We live modestly, but because we won't be working a "regular" job, our estimated budget includes savings, retirement, healthcare, 10% back to our organization, tithing, etc.

When I run my own budget numbers it looks like I'll need roughly $8k/month (not including taxes). Yet AI and the internet make it seem like that's twice as much as I'd need per month. So there seems to be a disconnect but I can't figure out what it is.

Also, the tax situation is something I haven't wrapped my head around yet. We will be taxed as Belgian citizens, but how should I factor that into our budget if I don't know whether my estimated budget is correct?

Any advice or suggestions would be greatly appreciated! 😊