The intensity/duration advice is good! I have typically replaced the final trot set with canter when adding in canter - starting with one lap each way, then gradually adding on. So the 30m trot isn’t 30m once canter is added. Have also used the Equicore. Also we only ride rehab horses in the arena and on good footing, to minimize risk of bad steps (no trail rides or walking out between the pastures).
Sorry I missed your question previously… I have been fortunate that I haven’t had any set backs or unsound days during the rehabs I have done once we got going - although if I did, I wouldn’t add on any additional trot/canter (depending on where you are), and would probably consult vet if I did. The first few days of trot on my mare were a bit awkward - she has a hinky trot to begin with and I was really worried she wasn’t sound, so I always had someone to watch when we first added in trot.
Typically, we have had the vet out (IIRC) about a week after starting trot and before starting canter to reassess, and again before jumping.
This app was recommended to me, and I’ll be using it if I have to go through this again: Interval Timer. If you change the interval name it reads out the directions (eg: change “Warm Up”, “High Intensity”, Low Intensity”, etc to “Walk Left”, “Change Direxction”, “Walk Right”, etc.)