AppMon Java Script agent collects the W3C Navigation timing metrics in all supported browsers by default, for calculating page loads.
Firstbyte metric: You can achieve the Firstbyte by configuring the Request measure in UEM.
For First paint, Please find the James(Dynatrace Employee) answer
Yes, you con consider it. but, keep in mind the time to first paint (when the first pixel has loaded) and visually complete are bit different. We are choosing the visually complete as it makes sense in verifying the User Experience at client browser level- since, it is the time taken for page to visually complete.