In my experience on the YM240, 1/3 throttle is all it needs on a cold-weather start. I used full throttle in the beginning as the Operator Manual recommends. The only difference that makes is a huge cloud of black smoke in the moment before I can yank the throttle back to 20%. Starting at lesser throttle makes minimal smoke.
But even using a 100% throttle start, there's time to pull the throttle back before it revs to maximum so I don't think this stresses it the way an automobile engine would go to peak rpm instantly.
Also - the manual says warm up at 1500 rpm. That sounds fast, like its hard on a cold engine. Just above cold-engine diesel clank, maybe 1200 rpm, sounds like less stress.
Last week I started the YM240 with the barn still below 40 degrees after it hadn't been started for weeks. First time in several years it didn't catch instantly. I think the battery was down to about 12.3 volts and I didn't run the Thermostart for the full 15 seconds that I should have. Second attempt after running the Thermostart until I heard the vapor burst into flame, started normally. (I always begin a first start of the day by spinning it with compression released to get oil pressure up, and on every start release compression while the starter turns the engine the first couple of revolutions).
I would have put the 2/10/50 charger on it, let it charge for a couple of minutes, and cranked with the charger set on 50 amp assist to make it spin faster, if that had been needed. Before I replaced the starter and injectors five years ago I sometimes had to do that in cold weather.
The more modern 3-cylinder YM186D starts easily like a car. I use the same procedures but they aren't really needed.