Bouncyness after rear suspension rebuild.
 

When hitting potholes (unavoidable in my area) in my 06 2.0T A3 FWD (140,000 mi), it felt like my car wanted to jump sideways. To remedy this, I bought this kit:

https://www.ecstuning.com/b-assemble...1k0512297dkt1/

The kit had:
rear control arms (upper and lower, with rubber bushings)
trailing arm bushings (polyurethane)
rubber spring isolators
rubber wheel housing bushings

I also purchased a pair of Koni Str T rear shocks with new oval shock mounts/bump stops/dust covers. These parts were all installed/aligned by a local suspension shop. While I initially thought it felt good, I find that the car bounces a lot, in a "floaty" kind of way that doesn't feel right. It's not like a up and down bounce like worn shocks, but a feeling like it never actually finally settles down. Before the rebuild it was more clunky. Now it feels different, but not necessarily safer or better. This morning I drove it to work in the rain and the ESC was going off every few seconds and constantly wanting to throw me off the road (wet, but not even driving through puddles). I took it back to the shop that worked on it and had them check everything and realign. Drove it back home in the rain and the ESC didn't trigger, but it still felt floaty/bad.

The wheels are stock 17s.

Is this floaty-ness a result of reusing the stock springs?
A result of fixing the rear and not the front (probably also needs work)?
A result of using polyurethane trailing arm bushings on bumpy roads (I feel like this is unlikely, as every other bushing is OEM rubber)?
Bad shock selection?
Something else?

Thanks in advance for any input.

If I were you, I'd look into the shocks first. The job of the shock is to keep the wheel in contact with the road. If it's not doing that, your ESC would be triggered.