You could just not use/bypass the MX and get full advantage of the speed. I know this seems weird, but many (maybe most?) home users just use whatever their ISP provides and maybe add a switch and some APs for larger spaces.
You could still leave your MX connected for learning/playing, but just not use it as your default gateway for traffic you want to get full gig speed for.
Maybe even create two VLANs to differentiate. VLAN 1 could get DHCP from and route through MX and VLAN 2 could also get DHCP from MX, but use default gateway of the Spectrum kit. Then you could pick and choose what/when to use each. I am not familiar with Spectrum, but if they are similar enough to Comcast and at&t I think you should be able to pull it off. It would be somewhat dependent on how the Spectrum kit works and how much control you have over it.