I like the idea of a household. I'm thinking of starting out a little more up north.
https://www.romanbaths.co.uk/roman-bathing-complex-0
Aquae Sulis sort of fits the parameters, it is a roman bathing complex with a temple to Sulis-Minerva. Sulis-Minerva was a goddess of healing which fits with Embie's character concept.
There could also have been a shrine to Mercury, patron deity of travellers, messengers and thieves, as the settlement was near the fosse Road. The Order of Mercury happens to be a forebear of the Order of Hermes in later centuries. Plenty of concepts to be had from that combination of traits.
Minerva was also the goddess of music and poetry so musicians wouldn't be out of place making it a good spot for Mari's character to hang around. Perhaps one of the buildings was a Taverna (inn) for travellers and pilgrims. Hospitality was a moral obligation for the romans.
Other possible character concepts, just to give some ideas:
*a druid who is seeking asylum. While the romans destroyed the power of the druids they still held respect among the local population. A shapeshifter, a blood mage, a weather controller. With the romans abandoning Albion he has to rethink his life.
*The bastard roman-Briton son of the roman Precept who used to oversee the settlement and who left his son in Britannia while he returned to Rome and his roman family. Maybe his magic is subtle, based on sacrifices to various deities. Luck favours the bold, an authoritive voice carries far and a blessing or a curse can turn the tide of battle. Maybe his father left him an ancient Wonder.
*An Oneiromancer half-fae with one foot in the Fae world. Dreams and oaths, a traveller of the Otherworld.
It was the religious settlement, rather than the road junction further north and the residential area now known as Walcot, which was given defensive stone walls, probably in the 3rd century. The area within - of approximately 23 acres - was largely open ground, but soon began to be filled in. There is some dispute as to whether these new buildings were private dwellings or were associated with servicing the pilgrims to the temple.
From the later 3rd century on, the Western Roman Empire and its urban life declined. However, while the great suite of baths fell into disrepair, some use of the hot springs continued. After the end of Roman rule in Britain around AD 410, some residents seem to have remained, but violence seems to have taken root.
So by the time our story begins the baths have fallen in disuse, the settlement is in ruins following an attack by Saxon raiders and only the temple settlement was saved to due to its defensive stone walls. The old guard has died or has left with the legions and travellers and pilgrims have have gone down.
For the Chantry I would suggest investing in:
*A Node. Maybe it's the sacred hot springs, maybe it's the gilt bronzed statue of Sulis-Minerva.
*A Library. A good sized library will have many benefits.
*A Sanctum: A ritual space that eases magic.
Other options: Cult, Retinue. Backup, Allies. Contacts. Wonder.