The Turris Omnia is a good option, though perhaps not for the prices I see listed today (I paid $150 a year or two ago). Runs a customized (still open source) version of OpenWrt by default, but you can load anything you want on it - it's basically just a small Linux box with WiFi antennas (and some other nice-to-haves like an SFP port, mPCIe slots, etc...). I used their official kit to upgrade from AC to AX, but you can also DIY if you were to purchase the older AC version of the router.

Link - https://www.turris.com/en/omnia/overview/