I use a refurbished Trendnet TEW-732BR running OpenWRT. It was powerful enough for my needs, energy efficient, atheros 9 based, and had a small desktop footprint. Many of todays routers are linux based and per the GPL, have to publish source.
Openwrt takes the source and provides regular updates and allows customization.
Mine has the https interface turned off and is only accessible via ssh. I added bandwidth accounting and a simple email program.
https://openwrt.org/
More recent devices are presently available refurbished
https://www.trendnet.com/store/produ...ed-refurbished