I've used this service before (to Aberdeen though), and always enjoyed it.
Prebooking further in advance is a big leap - it was only two months last time I booked (or maybe 90 days as you suggest), which is cutting it close if you want a berth at busier times of year.

I agree with atmosphere in the lounge on the train - really enjoying time spent there eating and drinking before sleep.

Making them all en suite in the future would also be a big improvement.