Android 9: OpenVPN Per App
How about "OpenVPN for Android" made by schwabe.
https://play.google.com/store/apps/details?id=de.blinkt.openvpn
https://github.com/schwabe/ics-openvpn

It has per app connection setting by whitelist or blacklist, though it doesn't support second VPN or SOCKS proxy for the excepted apps.