You are correct - now is not the ideal time to dethatch a zoysia lawn. The recommended time is in the spring and no later than about mid-July. https://extension.umd.edu/resource/planting-and-maintaining-zoysia-lawn

In general, we do not recommend for homeowners to mix zoysia and tall fescue, as they require different practices for fertilization, mowing, and planting technique. Creating a blend of the two types of grasses is only done for athletic feeds usually. It is a very specialized process for establishment and maintenance. If a blend of the two is something you would like to have, we recommend hiring a lawn care specialist who can do an on-site consultation with you, see what you already have in terms of zoysia, and if they can plug (or seed) more zoysia in with fescue, if that's what you want.

Here is information on choosing a lawn care professional.
https://mda.maryland.gov/resource_conservation/Documents/LawnCareService.pdf

Typically we recommend for homeowners to choose either a cool-season grass (turf-type tall fescue) or warm-season zoysia. There are pros and cons to both, which we have laid out here, https://extension.umd.edu/resource/starting-new-lawn.

You mention that you are dealing with erosion issues as well. If you are noticing problems with poor drainage or runoff during heavy rain events, you might also consider planting in some more deeply rooted perennial plants to reduce erosion where it is most problematic. 

Control Soil Erosion and Water Runoff
https://mda.maryland.gov/resource_conservation/Documents/tip3.pdf

Lawn Alternatives
https://extension.umd.edu/resource/lawn-alternatives

Christa