As @isoutamo already pointed out "migrate to Cloud" is a bit ambiguous term. As far as I remember if you're using Splunk Cloud, you still need to set up DS on your own (that's one of the uses for the 0-byte ingest license).
If you're simply migrating from on-prem installation to a cloud-based (GCP, AWS or any other provider really) Splunk Enterprise installation, you can put your DS there. A DS is not that sensitive to latency so it shouldn't matter that much.
There are other things to consider however - if your DS were to be exposed to the internet instead of connected to your network via VPN you will probably want to use deployment client authentication. You might also want to raise the phone home interval (which you normally probably should do anyway even with an on-prem setup) to lower load on DS and save on outgoing traffic (which you might be billed on in case of many cloud providers).
The amount of traffic will depend on the number of clients, phone home frequency, size of your apps and frequency of changes in apps.
You should be able to do some aggregation on splunkd_access.log events from your DS (there is a metrics series for the DS but it contains only the size of deployed apps so phonehomes don't count).
One caveat though - it will only contain the size of the payload as with the normal httpd access log. It won't account for all the lower layers - request and response headers, tcp overhead and so on.
