Those do look like marks on the negative vs the sensor — always worth having an air blower (e.g. the Giottos Rocket Blaster: https://www.bhphotovideo.com/c/product/259157-REG/Giottos_AA1900_Rocket_Air_Blower.html) and maybe an anti-static brush and gloves, regardless.
Dust may settle on your negatives while drying, and there are a lot of ways to avoid this — my trick is to run the how shower for a little while to steam up the bathroom where I develop and hang my film to dry: the moisture in the air helps reduce the dust particles floating around and also seems to help the final wash/Photoflo/et al drip off more cleanly.
You can always rewash your negatives, though if it’s dry dust my first suggestions should help. There’s a chance the marks on your example scan are from the developing process itself — are you mixing from a powder kit by chance? Or has the chemistry been used for a while/needs to be replaced?
Overall, even pro labs need to do plenty of spotting on drum scans to remove marks that couldn’t be removed any other way — so there’s always a certain amount of retouching needed no matter how clean your negs are.