Carma Bikes in Oviedo rents road, MTB and gravel bikes. Some friends have been going there for years so it’s a reputable shop https://www.instagram.com/...MXViaDBieG9qNTljcQ==
I’ve asked my LBS but they don’t do rentals.
There are other options for MTB outside of Oviedo (Valles del Oso). Nice to do with your wife https://www.turismoasturias.es/...erismo/senda-del-oso