We've enjoyed Hotel des Grandes Ecoles our last few visits to Paris. It's on a quieter street east of the Pantheon, easy walking distance to Metro and bus stops. It's also a pleasant walk from there to Rue Mouffetard or the Place Monge market.
The hotel is very quiet, in the midst of a small private garden. The staff is great, and the optional breakfast is very good (not a buffet.) We're booked into a superior double this coming June for €305 per night, breakfast included. The rooms are small, even the "superior" rooms, but charming and spotlessly clean.
https://en.hoteldesgrandesecoles.com/