Gare de Vernon-Giverny is not directly reachable from CDG. You’ll need to go through central Paris for this one with the RER (not included in the pass) but very doable, even with luggage. Just provide ample of time.

The destinations mentioned Amboise, Tours, or St. Pierre des Corps can be reached from CDG at some moments, but impossible to tell now for the 2025 timetable. Way to Early.

Today it’s possible to skip central Paris, but you might need to change trains somewhere along the, sometimes direct connection. But not very frequent. So you might need to wait a long time, before a suitable connection pops up.

Thank you @BrendanDB 

Depending on flight arrival time, we might have to take the RER to one of those gares to get better connection and then onward to Vernon-Giverny or Loire Valley gares or have to stay in a hotel in Paris then next day take the RER onward.

Normally I don’t mind taking the metro/RER (public transit) but of late I’ve read/heard thieves (et al) targetting tourists on the RER/metro. I am a bit concerned as they usually target seniors, immigrants of visible minorities etc. 

As long as you're careful, you shouldn't have to worry. There hasn't been a recent change : pickpockets have always and will always target lost tourists but as I said as long as you're careful and seem confident it's fine!