Can I set different Screen Time App Limits on different devices?

Is it possible to set Screen Time App Limits differently on difference devices, when the same iCloud account is used between them?

I would like to limit how long a child account can spend using a browser on iPhone, but have a more generous limit on a Mac Mini. However, it seems like App Limits are shared across devices - aforementioned child left the Mac logged in, which has used up all her App Limit allowance for the day.

Yes, but it's a trade-off. The answer is to turn off "Sync across devices" in Screentime settings. That way, all your App Limits can be different on different devices.

The downside is that it also stops syncing all the other Screentime settings, so you'll have to individually maintain the Always Allowed list, the Downtime hours, etc. Any changes that you make later will need to be made separately on each device.

It can be a bit of a pain, but it's probably worth it to get the flexibility you're asking about.