I could not make an 18" radius 00 spiral/ helix work on my 76" X 52" double bed size three level layout.
The layout plan shown is very full of track but so was my layout and mine could hold a 6 coach train and A3 so nothing wrong with plenty of track
However he plan has some nasty features,which I am sure would make it absolutely infuriating to operate as both main lines go through reverse curves through points at the station, quite unnecessarily in my view. Set track crossovers are just derailments waiting to happen. The bottom siding could easily be extended around the outside of the big curve ato merge into the present outer and the inner arranged to become a loop with a crossover simply by having tracks realigned on the diagonal so the main lines are in the middle, at the end of the diagonal changing on of the LH points for a RH at the crossover would throw the tracks one track width up the plan, it would need some but not much track cutting to achieve, which is why the planning software can't cope and produces something really ugly instead.
Set track is great, we take a box on holiday and create a layout around the holiday house (or even a premier Inn room! ) Thats what its for you can't pin flexi track to a premier inn carpet, well you can but you break the sleepers while you rip it up! and they don't like you ballasting the track. with PVA and granite ballast.
However set track curves stay curved and straights stay straight so for this size of layout I would use a mix of streamline points and set track track, 2ft streamline points for a crossover are shorter than set track and give a much better track spacing, you have to widen the track spacing on curves especially if you run MK3 s and Gresley Pacifics together but the 2ft streamline are so much less derailment prone than set track , 3ft is better and the short still 2 ft radius Y point a fantastic space saver in yards compared to set track.
What I do is use set track straights because the stay dead straight unlike flexi which likes to weave gently and curves, if you need non standard curves you can treat the curves like flexi track, cut the webs, ease them into larger radius, cut file snip the track ends level again, I ease 3d radius to 21" and 2nd to 3rd radius when I'm feeling mean and have a box full of 1st and 2nd radius, I also cut them up for short filler rails to suit pointwork or get the right angle or get the last bit of length form a siding etc.
Flexi is fine for big curves but gets dog leggy under 2ft radius, 3 ft when I do it, set track points are a derailment waiting to happen, I know, I made that mistake, the old Hornby diamond is diabolical.