With regard to the Xbox, I'm not really sure. I do know, like you've seen, that it can use data even when not playing anything. If you do decide to play games, though, aside from the data usage, keep in mind that real time games are not really going to work due to the latency of the system. There's up to a second or more of lag, which can cause real time games to be practically impossible to play.
If you are asking about scheduling the updates for Xbox, I have no clue if that's possible, nor if it is, how to do it.
With regard to scheduling updates for Windows 10, unfortunately, as far as I know, you can't do so with the settings available. The Active Hours setting only controls when the system reboots/restarts, but not when the updates will download. With it set to metered, you can manually download the updates at a time you choose, but you have to actively do it yourself, as, like I said, there's no way to schedule the downloads.
Now, that's with the controls available. There may be registry tweaks that will allow you to schedule the downloads, but I, personally, stay away from those, as one wrong move and it can screw the entire computer up. The registry isn't really something that most people should mess with.
I don't have my desktop set to metered, as it can only be done on a computer that is connected wirelessly. And though I can set my laptop and notebook to metered, I don't have them set that way. The Windows updates for me have never really been a problem, as I normally have a lot of data left, so I've not gotten to the point where I want to be able to control when they download. Right now, five days into my monthly allotment, I've used a total of 471MB of Anytime Data and 146MB of Bonus Bytes, and I have the 15/50 plan. If I get to a point where I am close to my data limit on a regular basis, though, I might change the laptop and notebook to metered so I can control them.
After a while you'll get used to how much data your programs, apps, websites and such use.
BTW, I forgot to mention, in addition to the antivirus program you have installed on your computer, Malwarebytes is very popular free antimalware program that a LOT of people use. Viruses and malware can actually use data, too, as well as wreak havoc on your system AND personal life if they get your personal data. The free version is all you need, and it's here if you want to download and install it.... https://www.malwarebytes.com/.
Now, just to let you know, for some odd reason that I have yet to be able to figure out, Malwarebytes seems to take FOREVER before it "finds" an update and starts downloading it. I manually update it about once a week, and sometimes it takes ten minutes or more of searching before it finds an update and downloads it. Why it takes so long is a mystery, but it might have something to do with the latency causing it to not "catch." You'll see what I mean if you install it. It doesn't affect the program, it's just annoying. LOL. Still, it's a great program to use to keep your computer that much cleaner. Anything that helps in that endeavor is a plus, as long as it's a known, trustworthy app/program. The updates aren't very big at all. Maybe ten or twelve MBs per update.
Lastly, I'm happy to help. I enjoy helping people to get the most out of the service as it can most certainly be downright frustrating getting used to a capped service, especially for those coming from an unlimited service. Once you get used to what you can and can't do, and how to do the things you can, you actually get to a point that you almost forget it's capped. This, of course, is with the exception of those who like to do a lot of streaming and/or play real time, twitch type video games. The real time games are pretty much impossible, but the streaming can still be done, within reason, that is, and in SD or lower resolution. The two aforementioned things are the things that have to, unfortunately, for the most part, be given up. But, even though we have to give those two things up for the most part, I look at the alternative, and that's dialup. No thanks. LOL.