Automated filesync cross-platform. Not git-style manual push pull. More of leave and forget and only intervene when there's a file conflict.

Unison has support for that on some platforms. On Linux, the option "-repeat watch" uses inotify to watch for changes and rerun unison each time there's a change. You can combine that with -auto and some of the options specifying policies for resolving conflicts (e.g. -prefer newer) to get a setup that works without user interaction.

Like Syncthing?

Also Resilio Sync, formerly known as BitTorrent Sync.

What the best way to do bidirectional sync using rclone?

rclone has a "mount" feature whereby you use FUSE to mount a filesystem which corresponds to a path in your remote share.