You are correct, all STE+ (including GM/PM/DM) in any class get access to any non-SkyClub SkyTeam lounge.
GM/PM/DM in W/J on DL also get access to Sky Clubs.
This policy is intricate for sure.
I think the only non-DL SkyTeam lounges are KE and AF/KL lounges in the US. For AF lounges, that's BOS (where it's in E and not very good), JFK (where it's in T1 so irrelevant), IAH, IAD, and SFO (currently closed). For KE lounges that's HNL, LAX, and JFK (T1).
I’ve been to the IAH AF lounge and it’s very small but quite good. Very nice change from SkyClubs. There’s also a VS Clubhouse nearby, but it’s only open for a few hours for the VS flight. Oh, and there’s a Saudia lounge, which I most certainly would never set foot in.