Well if you've tried one of the latest Kobo's and still find it's not up to the task, it seems that something like the Pocketbook Touch Lux (while not necessarily the fastest or with the best screen) is likely to be your best option for using with a large library due to the reasons you liked your old Pocketbook 360.
If you are still happy using your Nook for now, just keep an eye on any bargains for the Pocketbook (though unlikely since you have to import it, so you are always going to have to pay over $200
), or just consider if the additional cost is really worth it just to be able to carry your entire library with you.