I think the main question is which part of railway modelling do you enjoy most. You indicate that you have some advanced tools, suggesting that you enjoy "building", but also want to have fun "running trains".

Looking at 2mm fs, which is the only one I really know, the timing could be quite good. There's a small range of etched chassis just becoming available from the 2mm Association shops, including 3 which should suit your preferences (assuming that the GWR shirtbutton avatar is accurate) - Collett 22xx 0-6-0, 57xx 0-6-0 pannier, and 14xx 0-4-2T. There's also a brass chassis conversion kit for the Ixion/Dapol manor which should be available soon, and a similar conversion kit for the pannier in development. Also an etched chassis kit for the 45xx small prairie is available, and I believe one for the Dean Goods is under development.

I would also suggest considering the possibility of building a dual-era layout (or even triple-era), with some modern image stock (which for this purpose means diesel) which can generally be easily converted from N gauge RTR stock either by using the "drop in" replacement wheels or by doing it yourself with the lathe. This would give you some high-quality stock available to run almost immediately.

The one thing that you can't really get around in 2mm fs (apart from by subcontracting the job) is building your own pointwork - that's still on my "to do" list, so I can't give you any advice from my personal experience, but it seems that generally if you are willing to discard your first few attempts you will succeed in due course.

Looking a little wider than the 2mm fs field, I think within 4mm much the OO/EM/P4 discussion is largely similar to the N/2mm - do you want to get something running quickly (OO/N) or do you want to spend your time building (EM/P4/2mm)? I believe that there are similar alternatives within the 3mm and 7mm world too, but in both cases there is relatively little RTR equipment available, so it will be largely build-it-yourself whichever gauge you choose within those scales.

Your final comment indicates that you already appreciate one of the main pieces of advice I would otherwise have given you - start with something fairly small and simple, so that you can get it built. Ideally build in some scope for extension, but it's unlikely you will ever get the layout completely "finished".

Good luck with your project (specially if it's 2mm GWR!)

David