Ebike chain advice pls

[Ming the Merciless]

most used gear on his derailleur

A derailleur just shifts cogs, it only has jockey wheels (rear), no cogs.

 

[Anthony-C]

Wippermann Connex, but can only find their 8SE which is recommended for 7 speed ebikes with derailleur

... because 6, 7 and 8 speed chains are the same size. Connex should last for ages and an ebike chain in legs-only use probably more so. Here KMC explains the differences.