Not saying it's what's best for you, but I used flex for the straight areas and gentle 'fitter' curves and transition curves. On defined, dimensional curves (O72, O80, O96, et al) as defined/chosen for my layout plan based on RRTrack planner, I used preformed sectional curves. I've developed a nifty forming template technique for bending flex track. But for me, at my age, I chose the simpler approach for dimensional curves to speed things up.
BTW....and I'm sure you know this by now...you'll waste a certain amount of flex track in trimming the ends square after bending. That's the geometry of the stuff. So I did a cost comparison between the use of flex and pre-formed sectional track based on an O54 curvature. Using Gargraves MSRP pricing, a semi-circle of flex track will require 3 pieces...$26.85. Four pieces of Gargraves sectional O54 (a semi-circle) will cost $39.40. That's a difference of $12.55. But there's no waste for a complete semi-circle of sectional track.
So....how much is the bending, cutting, final assembly...and waste...of three pieces of flex worth to you to make a semi-circle of track? "Time is money" is the common cliche', Each of us has a different way of valuing our time...and skill.
That said, when I wanted to bend a piece of flex, I made a very simple bending template against which to pull the flex into curvature. I had a large piece of plywood...at least 4' in width...onto which I'd draw the desired curvature. Then I'd take a piece of lath stripwood....any strip of wood that is flexible enough to stand up to easy bending to the curvature....and fasten it to the plywood in the following manner.
I'd fasten several vertical standing screws into the plywood along the inside edge of the curvature line, about 6" apart, for at least the full length of 37" to accommodate the flex track piece. The screws would be long enough and stand high enough (below, say, the flat head) to allow the strip of wood to rest fully against the screw shanks. Then, bending the strip wood against these screws, I wood fasten two additional screws, one at each end, at the outside of the wood strip to hold it in place. There's your template!
So, then I'd simply take a piece of flex, place it to the outside of the template, keeping the ties flat on the plywood as I'd pull the track tight against the curved wood template. There's always some 'spring-back' of the curve; but if you're fastening the track down to a defined position on the table/bench work, you can easily pull the track into final alignment as it's fastened down.
I don't have any photos of this technique. It's been a long time since I built my 25'x35' layout. The plywood I used to create bending templates is long gone....having been 'peppered' with a bazillion screw holes. But it worked like a charm....for me, at least.
Gargraves has a how-to for bending their flex track on their website. Not the same as my technique, but I'm sure theirs has survived the test of time.
Hang in there! Gargraves/Ross...worth the effort and cost no matter which technique you use. AND....wholly American made!!!!!!