"my question is is this the best practice?"
No, but I would say there isn't a best practice, but it's a common practice.
A better practice might be to also use IPSLA, as also described by @MHM Cisco World .
"what will happen if the primary has an internet issue but the port is up?"
Your Internet traffic is impacted. Severity depends on what's the Internet issue. Could even be a "black hole".
"how does the cs know when to switch on to the backup link?"
When primary route path has a physical interface drop.
"Also, is there a way to use both links as load balance?"
Sure, have the two statics routes have same AD value. Understand this is "static" LB, i.e. really more load sharing then true LB, but that's true for most multi path routing.
Also, ECMP, using static routes, alone, has potentially same possibly impactful issues.
The only Cisco technology, of which I'm aware, that well handles Internet "issues" and supports (dynamic) flow LB, even using static routes, is PfR. Do know, dealing with Internet performance issues, TO you, can be problematic, even with PfR.
