Does this trip count for the commercial 300NM cross country requirement? I'm an instrument-rated PPL and recently flew (M20J) solo from NJ to Florida with one leg of 280NM and 3+ landings (not counting the return flight because I had 2 passengers).

Will this trip satisfy the commercial cross country requirement of 300NM solo when I start my training next year even though I'm not currently training for my commercial?
Yes.  It counts.  One leg needs to be at least 250NM and you have a 280NM leg.

§61.129   Aeronautical experience.
(4) Ten hours of solo flight time in a single engine airplane

(i) One cross-country flight of not less than 300 nautical miles total distance, with landings at a minimum of three points, one of which is a straight-line distance of at least 250 nautical miles from the original departure point. However, if this requirement is being met in Hawaii, the longest segment need only have a straight-line distance of at least 150 nautical miles;