If we rent a power aerator, and plan to aerate only once per year, would it be better to aerate now, before overseeding? Or wait and aerate in the spring before the bermuda comes back?

If overseeding this weekend, just drop seed and water.  Do NOT aerate or dethatch turf now as it will damage and / or severely injure the roots and crowns of the grass that will come back next year. Just mow low enough so seed can drop and contact the soil to grow.  Do those activities during June,July, August when bermudgarss is most actively and vigorously growing.