I think soloing is best left to instanced settings. Way too hard to balance open field sp where parties should be. Soloers are probably best placed inside a large mog house, with a scripted series of fights and tasks for their level over the course of 30 minutes or an hour, that gives them sp based on things that SE can script and control for their level.

Or put the parties in instanced scripted sp settings, and give the soloers the fields.

25-30 just sucks. Too low for 1 star R40 leves. Too big for the field regen rates. Mobs just aren't dense enough to do anything but leve link after about level 26. Where SP on the field goes from about 26k an hour to immediately half that, right at the time where your TNL's start getting 40 and 50k tnl. It's demoralizing.