I set the import start date to January 01, 2019 in GC and the sync module imported everything from that day. Same with the connected Withings account. It’s working fine for me. I don‘t use auto sync, I just sync manually. (macOS Mojave).
I setup the strava sync when I installed 3.5 pc version and it works fine. When I set it up I did it so gc would check for past 30 days of activities from strava ans automatically pull them in.