I can't confidently give any advice about the choke on an MC28, but,
when starting from cold, does it fire up almost immediately with a push start with the choke on? (as opposed to a kick start)

I have had a few dirt bikes over the years and when the rings get a bit worn, and correspondingly the compression drops, they exhibit the same symptom: Hard to kick start but once warmed up start easily.
You have a fairly new engine, so it won't be worn rings, but I wonder if it could be something else causing a similar effect?

I am just wondering, if in your case, a slow turning kick start is any different from a faster tuning push start.
If so, then maybe at low revs something like, say, a slight leak in the reeds might make it hard to start, but at higher revs and pressures, the dynamics make the leak is less of an issue?
If not, then investigating the choke seems like a good idea.
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