You have two master nodes? You'ld rather go with 1 or 3. With 2 masters, your etcd quorum is 2. If you lose a master, the API would be unavailable.
Now ... c5-xlarge should be fine.
Not sure why your console doesn't show everything (and not familiar with that dashboard yet). as a wild guess, probably some delay collecting metrics. Though you should see something, eventually.
If you use "oc describe node <node-name>", you should see the reservations (requests & limits) for that node.
Might be able to figure out what's eating up your resources.
Depending on what openshift components you're deploying, you may already be using quite a lot.
Especially if you don't have infra nodes, and did deploy EFK and/or hawkular/cassandra. Prometheus could use some resources as well.
Meanwhile, istio itself can ship with more or less components, ...
If using EFK: you may be able to lower resources requests/limits for ElasticSearch
If using Hawkular: same remark regarding Cassandra
Hard to say, without seeing it. But you can probably free up some resources here and there.