Heading to Europe Monday, with a 5 day stop later in June in Paris. The issue here is we arrive at CDG on a Saturday evening.

Is this the best (most cost effective) plan? I've thought of alternatives, but think this works the best, please let me know:

I'll top up my navigo decouverte for the Monday week at the airport, and get my husband a new one since his fell in between turnstiles in Paris in 2019 and he couldn't retrieve it!
Will also get each of us a 1 day, 5 zone Carte Mobilis to be used on Sunday, because, weather permitting, I want to finally go out to Yerres to the Caillebotte estate. (We were headed there in 2019, but the yellow vest protests held us back).

Taking a taxi from the airport, because after 3 weeks of travel, a 9 pm arrival, and a broken toe, I don't want to RER then walk it to our hotel.

Sounds like a reasonable transportation plan to me.

You may load a Navigo Jour pass (one day, unlimited use, zone 1-5 at 17.80€) onto a Navigo Découverte card. However, a ticket on the RER D to Yerres is 4.45€ each way.