I think IES lights will help a lot for realism, which is quite important in your case!
On the subject of the sampling rate, you can try to boost more the value, but i don't think it will help more than it already is.

With an afterthough, i have a more few points that you can try to further ...

I would suggest a few thing that you_ didn't try :

- Setup your lights with the a Sample Rate value :
Sample Rate

The sample rate determines how many samples will be used in the shading of the light being cast. In some cases, there may be more than one light in your scene, and some of them may ...

I'm using Octane now with great satisfaction for the last few years to create architectural visualizations.
Exterior scenes render really easily noise free in a very short time. It's fantastic.
However, I have difficulties creating noise free interiors where there is limited 'natural' light hitting ...