Achtuning is really, very good. Top tier staff and super helpful. I know you're limiting the radius to Eastside, but Stone's German Garage on the Westside is so excellent. Awesome shop, great guys that shoot it straight. A bit more affordable than Achtuning, too.
Stone's German Garage - Home (stonesgermangarage.com)
I would also recommend searching up Avants -- it's a car club in the area. They charge a $99 yearly fee, but the perks and discounts more than make up for it. The events they put on are amazing!
Avants - Drive Everything
Edit: Saw that you're mainly concerned with a shop that installs mods. Not sure if Stone's does that; they're more of a traditional mechanic shop.
If you wanna save a bit of money on mods installs, definitely check out Stew's Self-Serve Garage in Kirkland. Hourly lift rentals with access to every tool you'll need. The guys that run the place are more than happy to answer any questions at all while you're getting your hands dirty. Avants members get 15% off, too