For a thread of the conversation on the same topic I had in Mastodon you could follow this link, surely you will find the activity and the links very useful.

I’m planning to do it genuinely hard sci-fi, so it will have to deal with rather complicated things like those 5.4 Sv of surface radiation (see Colonization of Europa - Environment).

As for the player characters I’m planning to have a crew of scientists and, probably, some military types.

Yes, the sub will need to be very strong and prepared to deal with depths of 100 km Europa - Subsurface ocean.

Interesting, there could be traits or backgrounds related to a given origin or birth place for example (something that D&D 5 and other games already do).

A poor game session could be the result of many things (not necessarily related to the game), like the mood of the referee or the players at the moment of playing or if the referee have had enough time to do a proper preparation.