Did more rendering tests and I think I found a way out also for this PFX issue.

Converting the big Paint Effects Tree Brush Type from [Mesh] to [Paint] seems to solve the problem.
Well, now I guess I can leave MayaBatch rendering through whole night unnatended without worrying about crashes.
The memory usage when batch rendering doesn't seem to reach more than 1GB until now, and it looks very stable.

As for this, would it be right to conclude that [Mesh] Brush Type can cause RAM usage issues?

Hope this might help others just as it helped me.

Thanks!