Paul
My preference is also for Helicon Focus. I use it all the time when photographng any of my models. In fact,I leave my Canon 80Don the tripod all the time because of my need for focus stacking.
David
CombineZP is/used to be freeware, and it works very well. Apart from that, long shutter time and stopped down to near F10 or higher, and place the camera on a holder or support where it will be absolutely still for the duration of the exposure.
I’ve been photographing models for over 30 years now. Got my depth of field when using film cameras with a 28mm lens to which I added a home made pinhole aperture. Had a lot of success with that setup, many photos published in magazines, with 9 cover shots and a first place in MR’s annual photo contest.
Switching to digital meant this setup didn’t work very well, but discovered that I could use a wide angle zoom lens (11-18mm) to get almost similar depth at 11mm and f/22.
I have all kinds of tips, hints and examples of model railroad photography in my website, in my signature.