I can only get the cheap-ish business class fares (i.e. between 300-350 in each direction), if i choose XXX-CPH-ARN-HEL (where XXX can even be AMS or BRU) but none of the other finnish airports.

On the one hand it's kind of annoying that these fares do not show up on google flights (any specific reason why not...?), on the other hand you can also book them oneway