I'd say go for the Sony A7II. (Ideally, A7 if you want small)
1. You get to use your Pentax lenses on it too.
2. FF (since you mentioned Shallow DOF look as a preference)
3. You can adapt many other lenses like small RF ones.
4. You can always still go with the apsc E-lenses if you want cheap lenses.
While m4/3 has caught up a lot since the days I used it, I don't really like the working distance if I want the same shallow DOF for isolation effect.
Yeah..yeah.. they do work with the faster, longer lenses and small subjects....
But the working distance (which is seldom mentioned) is not as easy.
Especially so once you move to half, 3/4 body and full body shots.
The 45/1.8 for example becomes a 90mm equivalent FL, so to cover a full body shot, you'd have to stand roughly twice that of what you do with a 50mm on FF, also losing out on the DOF as a result.
Not to mention that its sometimes not possible to stand any further.
If you keep to half body portraits and small objects, m4/3 is ok to get a shallow enough DOF "look" though.