I don't know much about music for the Assumption, but there are some good pieces based on Marian plainsong in the Nibelle's collection of 50 pieces here (go to collections):
https://imslp.org/wiki/Category:Nibelle%2C_Henri
The final piece - Ave maris stella ends with a short toccata which sounds more difficult than it is (though my small hands don't like the stretches).
There are quiet pieces: Ave maris stella, Ave Maria and Salve Regina among the Offertoires and Communions.
Of course there are the Ave maris stella pieces in Dupré's Vespers: https://imslp.org/wiki/Vêpres_du_commun_des_fêtes_de_la_Sainte_Vierge%2C_Op.18_
And his beautifu, simplel Salve Regina (a favourite of mine): https://imslp.org/wiki/8_Short_Preludes_on_Gregorian_Themes%2C_Op.45_(Dupré%2C_Marcel)
These are all copyright, of course, but easily available.