The sync option on Strava was added recently and means if you sync with Connect it will automatically upload to Strava (though won’t set the title or the bike you were riding).
I think it’s all done from Connect, so you’d have to go into their site and try to disable. I think there’s a connected account setting somewhere and you can remove Strava from it.
You can then upload in Strava using the plugin or just manually uploading the .fit file. Either seems to work okay for me, although the Connect sync works too. However I strongly advise not installing Garmin Express to sync to Connect. I’m convinced that was what was corrupting .fit files for me.