You can't buy a mass-market computer that doesn't have either an obvious backdoor or proprietary firmware right now, except for the one Chromebook that Libreboot supports.
And even then, your RAM has proprietary code running on it (I'm not aware of any DDR4 that doesn't have embedded SoCs for initialization, none of that is done in CPU firmware anymore), your hard drives can have up to entire embedded SoCs with proprietary code running on them, and there isn't an unencumbered 802.11AN wireless chip in existence. The only bright side to that is none of that hardware has system-wide access the way these backdoor coprocessors do.
And even then, your RAM has proprietary code running on it (I'm not aware of any DDR4 that doesn't have embedded SoCs for initialization, none of that is done in CPU firmware anymore), your hard drives can have up to entire embedded SoCs with proprietary code running on them, and there isn't an unencumbered 802.11AN wireless chip in existence. The only bright side to that is none of that hardware has system-wide access the way these backdoor coprocessors do.