There are times when the drysuit is too warm (above 23 degrees Celcius, sunny, and no wind). The best clothing in that weather depends on the water temperature. If the weather comes at a time when the water temperature is high enough, then I paddle in rash guards (I am not so fond of being burnt by the sun). But if this happens in spring time, when the air temperature is much higher than the water temperature, then rash guard clothing should be restricted to when you know you can easily get up on land, it does protect a little against cold from water, but only little.
As an alternative, I have 0.5 mm neoprene pants and shirt. I see the usable period for them as an alternative to the drysuit or rash guards as very limited, except for the shorts. The thin neoprene pants gets similar to a wetsuit experience. Just putting the trousers on will mean that you are wet from sweat when you get back in. This is, I believe, not due to me having bad quality (I use NRS 0.5 mm neoprene apparel called Hydroskin), but due to the lack of breathability. The 0.5 mm material is doing a better job than I would have expected of protecting you from the cold of the water, thus if you are not using a drysuit, they offer protection, and are a good alternative to a wetsuit. But they are not doing a good job of protecting you from sweat.
What I have found is that at times when I would wear only rash guards, but risk getting a wet seat (launching from beach, playing in the water, high waves and winds with spray,...), then I enjoy the Hydroskin pants, or if it is warmer, the hydroskin shorts, still using the rash guard shirt unless water is too cold for that.
So I have become quite fond of the hydroskin shorts for paddling in warmer days with warm water. The rash guard pants are more comfortable if you are sure to have a dry seat, but at any chance of a wet seat, I head for the Hydroskin shorts.
I also prefer the hydroskin pants for days in cold water where it is so hot that the drysuit is not a good choice. However, there is very few occasions where I can see the thin neoprene shirt as a better alternative than the drysuit or the rash guard shirt. Those occasions come in spring and early summer, so maybe next year I have found more occasions to say positive things on that.