Snorlax - Shadow Snorlax actually performs worse than non-Shadow Snorlax though not by much. If you really want to use Shadow Snorlax you’ll want to keep an eye out for another event that lets you TM away Frustration.
Ralts - Gardevoir is okay in GL. It’s Mega evolution is fantastic though we don’t know when if ever Mega evolutions will be allowed in GO Battle League and since the CP sky rockets when you Mega evolve it might not be eligible for GL. But you can always make one for GL now and when the Mega evolution is released power it up for use in raids.
December CD - In the past all Pokemon that had a Community Day could learn their exclusive move through evolution during the December Community Day. If Niantic continues that trend for this year then yes, you can evolve your Bulbasaur to Venusuar and get Frenzy Plant. For Shadow Pokemon though you need to make sure Frustration is TM’d away otherwise it wont learn the Community Day move and you can only TM Frustration away during certain events. We don’t know if Niantic will have another one of those events before December CD.