Your best bet, if you want to face an alien ship, are the Barnacles. The alien ship (note: Not necessarily a Thargoid) appears there in irregular intervals to "harvest" (as it seems) the Barnies.
To enhance your chances for an alien encounter, you might consider heavy mode switching between Solo and Private Group(s). I had two encounters yesterday evening, after a lot of mode switching. In my case, every encounter followed close to the log-in/during approach, so i am not sure whether waiting at the site actually helps.
Oh one encounter followed directly when I approached the Barnacle site (shown as "Unknown Barnacle" in you Nav panel).
System/Planet/Coords:
Pleiades Sector OI-T C3-7 A 6
-42.7785 -21.7142
Details:
https://forums.frontier.co.uk/showthrea ... ost5575815