I downsized to 17" wheels, granted for different reasons, but it definitely rides nicer on them. I switched back to my 18s for winter and have been cursing them over bumps haha. Koni Special Active dampers might help too, I have those paired with eibach springs on my car and most of the time it feels more comfortable (the exception being huge bumps), I think with stock springs you'd have a really comfortable combo.
As far as comfier manual cars go, I have a B8.5 A4 6MT as my daily/commuter and I love it. even on big 19" wheels with factory sport suspension it's far more comfortable than my GTI. If you were to go A4, I'd recommend the gen after mine (B9) you could get a 6MT for 2017 and 2018. But there really isn't much else out there.
RE: M2 Comp, I have driven a few of these and the GTI (to me anyways) is definitely more comfortable, but darn the M2C is an amazing piece of kit haha