First, why do you keep referring to an ONT? I'm sure you've done your research, but it reads like you don't have fiber-to-the-house (FTTH). If you don't have FTTH, you don't need an ONT. An ONT connects to a fiber cable from Cox.
If your question is can a DOCSIS 3.1 deliver gigabit speed? Yes, it can. A few contributors on the Forum have measured 900+ Mbps. That's about right considering network overhead with TCP/IP (-10%). Whether you get gigabit is another story.
If you haven't previously had Cox Voice...or any provider of VoIP (packet-switching phone service)...then you'd need a tech at your house to physically rewire the demarc for your landline. Other than the rewire, you wouldn't need a tech to activate your modem, cable-box or telephone modem (eMTA). It's basically plug-and-play...then troubleshoot.