I am having a similar issue, so maybe we can find a solution together. I have a Suzuki 2021 DF25A tiller model that I purchased new from onlineoutboards. The motor has maybe 10 - 15 hours on it at this point. I brought it in late Summer of last year. Our local dealer replaced the water pump saying it wasn't cooling properly and that the engine was going in to "limp" mode because of this. Well, shame on me for never getting back on the boat after it was "fixed" last year. Had it out this past week and has the exact same problem, so nothing was fixed.
Hoping someone can point me in the direction of getting this repaired. I read in another forum that it could be a bad "Neutral Throttle Switch." I will probably buy the part and swap it out to see if that fixes the issues. My symptoms are roughly the same as the OP's. Starts fine, first or second pull always. Idles nice, you can rev up and get on plane for a few seconds and then it will start to sputter and the revs are limited. I can't get any more than about 1/3 throttle.
Any ideas?