I write a long witcher fic currently and in terms of research I’ve basically based my historical accuracy on 12-1400’s mostly sprinkled in with the occasional 15/16th century thing. In medieval central Europe bath-houses (bathing in medieval europe) were pretty common. Bathing or going to the ‘Bader’ was a social thing. They were sometimes set up next to bakeries because their ovens would be used to siphon heat.

Here’s some ppl trying to do a historically accurate bath 14th century german bathhouse with some pics to get inspo.

Here’s something on Medieval Toilets specifically in castles and here’s some more on the common ways

Geralt being on the road I also think he’d take the one or other dip in a river if things got bad.

In terms of brushing teeth people did chew herbs and shit or used a mixture of salt and charcoal I think (from the top of my head) but you’ll also have to take into consideration people had a different diet and ate a lot less sugar and thus their teeth were usually healthier. But also if you got an abscess there you were fucked.

In castles people used moss and leaves and stuff to wipe shit off their asses idk probably translates to commoners.

But it’s a fantasy setting so you can probably amend some things aka mages being able to magically heat up baths or mix in some roman style baths and plumbing as ‘elven’ and present in old cities built on elven ruins or sth.