Currently the buses are easy to board without a ticket but not all the métros nor the RERs. I noticed recently that the turnstiles at St Lazare required a ticket to enter for the métro 14 which still operates normally as does métro 1 as these lines are fully automated. If you are coming from CDG and wanting to take the RER, I would charge my Navigo Découverte card for the week.
Most museums are open but staying near rue Cler means a lot of walking with a limited number of buses serving the area and practically no service on métro 8 for Ecole Militaire.
I would download and use Citymapper for public transportation information and download apps for VTCs such as Lecab, Snapcar, Kaptan, and Bolt. Uber has been using not only surge pricing but dynamic surge pricing meaning the actual price may not be known until you get your final bill.
Fares between CDG and Paris are fixed at 50€/55€ for the Right/Left Bank respectively. If you call for pickup, there is a 4€ upcharge. The additional charge for a next day reservation is 7€ but there are no other fees (for up to 4 passengers) when using taxis. From the 7th arrondissement to CDG, calling in advance for pickup at a later time, the fixed cost is 62€ (55€ + 7€).
For the Taxi G7 English language dispatcher: 01 41 27 66 99 -or- +331 41 27 66 99