The Cromite GitHub page also says it supports arm32-v7a right at the beginning (for some reason later in the README it lists the platforms again without it, someone probably forgot to change it), I believe it’s the file in the releases called arm_ChromePublic.apk (not arm64), so try that

(Note: it does say it’s Android 7 or higher though, so it still might not work unless you try some custom ROMs)

Unfortunately, it’s not open source though.

SteamOS 3 hasn’t been released as an ISO (yet?), but there’s the unofficial HoloISO as a replacement.

I couldn’t find any indication of it being open source anywhere