We are in the same boat - only our pasture has a lot of weeds, in addition to native grasses, orchard grass, some timothy, fescue and clover.
Our plan is to mow and bale the field to clear it off and then spray to kill everything and reseed. We have a sprayer, but our local ag store (Southern States) will spray it too - if you don't have the equipment. They also offer a seed drill for rent - which we plan on renting.
At the end of the day, the plan is to have two separate fields. One being 100% orchard grass and the other 100% timothy.
Several reasons for this. First one is - they both come-in at different times. Orchard grass and a few weeks later, the timothy. This gives us a chance to focus on one field and then the other without having everything down at the same time. All of our eggs aren't in one basket so to speak.
The second reason is hay quality (as I understand it). If we have a mix of orchard grass and timothy - if we cut when the orchard grass is ready, the timothy hasn't had a chance to maximize it's yield. If we wait on the timothy, the orchard grass will have gone past it's prime cutting point, would likely have gone to seed, be stems and not as desirable hay.
I guess if we ultimately like one grass over the other, we would make both fields the same.
Take what I say with a grain of salt - talk is cheap, we'll see how our fields turn out, but we have a path we are holding to.